Chapter 3: The Zemanim-Zeman Kerias Shema & Zeman Tefila[1]

Zman Kerias Shema:[2]

It is a Biblical command for men to recite Kerias Shema every morning.  The following will discuss its start and end times:

 

1. Start Time-Mi Sheyakir and Alos:

Start time-Mi Sheyakir-Lechatchila:[3] Rabbinically, the start time of Shema is from Mi Sheyakir, which is defined as when there is enough light in the sky to recognize an acquaintance whom he does not see very often but does see on occasion, from a 4 cubit distance.

Start time-Alos-Bedieved and time of need:[4] Bedieved, if one read the Shema prior to Mi Sheyakir, he nevertheless fulfills his obligation, and is not required to repeat it afterwards. The same applies even initially in a time of need.

 

1. Lechatchila time-Before sunrise:[5]

It is a Mitzvah Min Hamuvchar to Daven Vasikin. This means that one is to recite Shema and its blessings slightly before sunrise and then Daven Shemoneh Esrei immediately by sunrise. Even one who is not Davening Vasikin, it is proper for one to recite at least the first paragraph of Shema before sunrise, even though he does not plan to Daven Shemoneh Esrei at sunrise and is not wearing Tallis and Tefillin, and will thus only Daven later on.

1. After sunrise-Best time to say Shema after sunrise:[6]

Within Shacharis: If one did not read the Shema before sunrise, then he is to do so as soon as possible after sunrise within his prayer of Shacharis, and thereby is to Daven by the first available Minyan after sunrise. [Thus, if one has a minyan available at both 7:00 and 8:00, then it is preferable for him to Daven at 7:00 even though it is already past sunrise and the second minyan at 8:00 will likewise also finish the Shema before the end time.]  

Prior to Shacharis before Sof Zeman: When saying Shema after sunrise, he is not to say Shema prior to Shacharis with intent to fulfill his obligating as soon as possible after sunrise, unless he will be saying Shema within Shacharis past Sof Zeman Kerias Shema, in which case he is to say it as soon as possible after sunrise, as explained next.

Prior to Shacharis after Sof Zeman: If one will not be able to recite Shema within Shacharis prior to the end time, then he is to recite it prior to Shacharis as soon as possible after sunrise, if sunrise has already passed, with intent to fulfill his obligation. He is to recite the verse of Shema, Baruch Sheim, as well as the first two paragraphs of Viahavta and Vehaya with intent to fulfill his obligation, and if he suspects that he will not even recite Hodu within Zeman Kerias Shema then he is to also recite the paragraph of Vayomer with intent to fulfill his obligation of remembering the Exodus.

Saying Al Tenaiy if one is unsure if he will say Shema before Sof Zeman:[7] If one suspects that he may not be able to recite Shema within Shacharis prior to the end time, then he is to do as above and read the paragraphs of Shema beforehand, although he is to recite it with a stipulation that it only stands valid if indeed he will be unable to recite it later on in Shacharis within the time.

1. Sof Zeman Kerias Shema-Until what time may the morning Shema be recited:[8]

The morning Kerias Shema must be said within three hours into the day.

How to calculate the three hours-Understanding the Luach: The three hours begin from the start of the morning. It is however disputed amongst the Poskim as to when the morning begins in this regard. Some Poskim rule that the day begins from Alos Hashachar/Daybreak. This approach is commonly known, and published, as the time of the Magen Avraham for the morning Shema. Other Poskim rule that in regard to the three-hour calculation, the day begins from sunrise, and hence the morning Shema may be recited until this time.  This approach is commonly known, and published as the time of the Baal Hatanya and Gr”a.

The final ruling: The morning Kerias Shema must be said within three Zmaniyos hours from sunrise, which is the later time which is known as Zman Baal Hatanya/Graz and Gr”a. Many, however, are stringent to initially recite it within three Zemaniyos hours from Alos Hashachar, which is known as the earlier time of Magen Avraham.

Saying 45 minutes before end time: Even those who follow the former approach, as is the Chabad custom and custom of many, are to endeavor to always recite it at least 45 minutes before its end time, which quite often falls earlier than the end time of Magen Avraham.

1. The law if one did not read the Shema on time:[9]

Although the time of reading the Shema expires after the third hour of the day, nonetheless, if he did not read it by that time, he should nevertheless still read it to accept the yoke of heaven upon him. It may be recited at any time of the day for this purpose.

Zeman Birchas Kerias Shema:[10]

The start time: The start time of reciting Birchas Kerias Shema begins at the same start time as that of Shema, which is Mi Sheyakir, and Bedieved from Alos.

The initial time: Initially, the blessings of Shema are to be read at the time that one fulfills his obligation of the morning Shema and not afterwards as explained in Halacha 1C, and hence ideally, the Shacharis prayer is to be recited at the time that one desires to fulfill his obligation of the morning Shema. Practically, see Halacha 1A-C regarding the initial time for when the Shema is to be read, and consequently as to the initial time that it is to be read together with its blessings within the Shacharis prayer.

The end time-Reciting the Shema with its blessings up until the fourth hour of the day: Although the time of reading the Shema expires after the third hour of the day, nonetheless if he did not read it by that time, he may nevertheless still read it together with its blessings up until the fourth hour of the day.

Reciting the Shema with its blessings past the fourth hour of the day: Some Poskim rule that one may no longer read the Shema together with its blessings if it is past the fourth hour of the day. Other Poskim, however, rule that one may read the Shema with its blessings anytime of the day up until sunset. Other Poskim rule that it may be recited up until midday. Practically, although the main opinion follows the first opinion, nonetheless, the custom of world Jewry is to be lenient and permit the recital of Shema with its blessings up until midday just as we permit regarding Shemoneh Esrei of Shacharis.

Tashlumin if one did not read the morning or evening Shema:[11]

If one did not read the morning or evening Shema then it is disputed amongst the Poskim as to whether one can make it up by the next Shema period just as is the law regarding Shacharis. Practically, the main opinion follows the approach that it cannot be made up. [Nonetheless, there is nothing to lose in being stringent and repeating the saying of Shema as Tashlumin after Shacharis or Maariv, and in worst case scenario, it will be considered as if one read from the Torah.]

  Zeman Tefilas Shacharis:[12]

According to all opinions, the times of prayer is not Biblically mandated and one may pray to fulfill his obligation whenever he chooses. The set times of prayer are merely Rabbinical.

The start time-From when in the morning may one daven the Shacharis Shemoneh Esrei?

Neitz/Sunrise-Lechatchila/The initial time: The ideal time for praying Shemoneh Esrei is exactly by sunrise, [unless a minyan is not available at that time, or if Davening this early will compromise on one’s Torah learning schedule, or ability to concentrate in the prayer, in which case one may Daven later on]. Thus, one should not Daven Shemoneh Esrei before sunrise unless it is a time of need.  

Mi Sheyakir and Alos-Time of need and Bedieved: In a time of need, it is permitted for one to Daven Shacharis prior to the time of sunrise, but after Mi Sheyakir, and in certain extraordinary cases it is permitted to Daven Shacharis even prior to the time of Mi Sheyakir but after daybreak/Alos. In all cases, Bedieved, if one Davened Shacharis from after daybreak [but before sunrise], then he has nevertheless fulfilled his obligation.

B. Cases of need in which praying before sunrise is permitted:

Traveling before sunrise: If one must travel out of the city prior to sunrise and will be unable to Daven afterwards while standing, then one may pray the entire prayer before sunrise. [Thus, if one must take a bus or flight prior to sunrise which will continue until after Zeman Tefila, then he may pray the entire Shacharis prior to sunrise after the time of Mi Sheyakir.]

Must begin work prior to sunrise: If one begins work very early, near sunrise, then he is to Daven before going to work. This applies even if he will have to Daven alone before sunrise without a Minyan. Certainly, it is permitted for a group of men who go to work before sunrise to arrange a daily Minyan prior to sunrise.

No Minyan after sunrise: If the only available minyan is before sunrise, then it is better that one Daven with the Minyan before sunrise than to Daven alone after sunrise.

One suspects that he may miss Zeman Tefila:  In any case that one suspects that delaying his prayer past sunrise may cause it to be delayed until after Zeman Tefila, then he may precede to Daven prior to sunrise.

C. The order of prayer when praying prior to sunrise:

Before Alos: One may recite all the morning blessings that he became obligated in, and may recite Mah Tovu etc, up until the paragraph of Terumas Hadeshen. In the winter, he may recite also the paragraph of Terumas Hadeshen prior to Alos.

After Alos but before Mi Sheyakir: One may recite all the Karbanos, as well as Pesukei Dezimra without Tallis and Tefillin.

From Mi Sheyakir: One may put on Tallis and Tefillin and recite Shema and Birchas Shema and Shemoneh Esrei.

  The end time-Until what time may one Daven Shemoneh Esrei of Shacharis:[13]

Sof Zeman Kerias Shema-Initially Davening Shacharis before Sof Zeman Kerias Shema: Initially, even after sunrise, one is to try to Daven Shacharis as soon as possible, and is initially to try to Daven prior to the end time of Shema, and fulfill his Shema obligation at this time.

Sof Zeman Tefila-Davening Shemoneh Esrei of Shacharis before Sof Zeman Tefila: The time for Davening Shemoneh Esrei of Shacharis extends 4 hours into the day which is up until 1/3 of the day. Thus, if one is unable to Daven prior to the end time of Shema, then he must initially complete Shemoneh Esrei prior to Sof Zeman Tefila. Past this time, it is no longer considered morning, and it is hence no longer the time for Davening Shemoneh Esrei of Shacharis. The entire Shemoneh Esrei prayer must be completed prior to the above-mentioned time arriving.

How to calculate Sof Zeman Tefila: The four hours are calculated beginning from sunrise.

Chatzos-Davening Shemoneh Esrei of Shacharis before Chatzos: If one did not Daven Shemoneh Esrei prior to Sof Zeman Tefila, whether due to negligence or matters beyond his control, then he may nonetheless still Daven until the time of midday.

After Chatzos-Davening Shacharis after Chatzos: Once it is after midday, one can no longer Daven Shemoneh Esrei of Shacharis, unless he does so as Tashlumin immediately after his Mincha prayers which he can only begin Davening from a half hour after midnight. One must at least begin the Shemoneh Esrei prayer prior to midday. If one began Shemoneh Esrei before midday then he may continue it past midday.

Must women Daven Shacharis within Zeman Tefila? Women are required to Daven Shemoneh Esrei of Shacharis within the time of Zman Tefila just like men.

  Cases of allowance for praying after Zeman Tefila or Chatzos & Chabad and Chassidic custom:[14]

Chabad and Chassidic custom: As received in tradition from the Chassidic Masters, it is permitted for one to extend his prayer past the end time if pressuring oneself to complete the prayer before the end time will compromise on one’s level of concentration and ability to emotionally bond with G-d during his prayer. It is clear that even amongst members of the Hasidic movement, there is no excuse to delay praying on time without the above-mentioned justifications of necessary concentration and emotion. Thus, if the prayer will have the same quality or lack of quality both before and after the end time, then certainly there is no justification for delaying it past the end time, and it is forbidden to do so even for the sake of Davening with a Minyan, as stated above. [The notion that some people have that the concept of Zeman Tefila no longer exists and they may compromise on it for any reason they seem fit, and the Halachos regarding it are obsolete, is a grave error and causes the great Divine prayer service of the Hasidic Masters and their followers to nurture their transgressions.]

Past Zeman Tefila: It is permitted for one to delay prayer past Zeman Tefila for the sake of having greater concentration in the prayer. Thus, if one has a headache, or is very tired, and will therefore be unable to concentrate on his prayers, then he may delay it past Sof Zeman Tefila for the sake of praying with greater concentration. Likewise, it is permitted to delay praying past Zeman Tefila for the sake of studying the Chassidic teachings and contemplating the greatness of G-d, in order to pray with greater concentration and emotion.

Past Midday:  According to the ruling of the Shulchan Aruch, one cannot Daven Shacharis past midday, and the only thing that he can do is to Daven Mincha twice, the second time as a Tashlumin for Shacharis. However, some Poskim defend the recital of Pesukei Dezimra and Birchas Shema and Shemoneh Esrei even past midday. Practically, those who Daven Bearichus and begin praying a while before midday and have their lengthy prayer extend past midday have upon whom to rely and so is the encouraged Chassidic practice to not compromise on praying in length due to the midday limitation. However, those who delay praying past midday without any good reason, may not be lenient to pray after midday, and should Daven Mincha twice if he was not negligent in his delay.

  Minyan Versus Zeman Tefila-Is it more important to Daven with a Minyan or to Daven within Zman Tefila?[15]

Davening within the time period that the Sages established is more important than Davening with a Minyan. Thus, if one is Davening with a Minyan who will not reach Shemoneh Esrei before the conclusion of the Zman Tefila, it is better that he Daven in private in the Shul, rather than delay until after Zman Tefila to Daven together with the Minyan. In such a case, he is not required to walk out of Shul to Daven Shemoneh Esrei.

The Zeman for Mincha:

5. The start time of Mincha:

Mincha Gedola versus Mincha Ketana:[16] One who Davens Mincha from 6 ½ Zmaniyos hours into the day [i.e. half an hour after the Halachic midday], and onwards, fulfills his obligation. This time period is commonly referred to as Mincha Gedola. However, initially, one is not to Daven Mincha by the time of Mincha Gedola, and is rather to Daven by Mincha Ketana. Nevertheless, some are accustomed to Daven by Mincha Gedola due to scheduling reasons, and certainly one may do so in a time of need, such as to catch a Minyan or so one can eat a meal or if one is traveling and the like.

At what time may one begin to recite Karbanos for Mincha?[17] The Karbanos for Mincha are not to be recited prior to 6.5 hours into the day, which is 30 minutes after midday, and the earliest legal time in which Mincha may be Davened.

1. The end time of Mincha:[18]

Sunset: The prayer of Mincha may be recited until evening. It is disputed amongst Poskim as to the definition of “evening.” Some Poskim rule it is permitted even initially to Daven Mincha past sunset, up until Tzeis Hakochavim.  Other Poskim rule one may not Daven Mincha past sunset. Practically, initially, one is to Daven Mincha before sunset, although one is not to protest against those who are lenient to Daven Mincha after sunset, until Tzeis Hakochavim. This applies both on Shabbos and during the week.  In a time of need, such as in the short winter days, one may be lenient to Daven Mincha after sunset, up until nightfall.

Is it better to Daven Mincha in private before sunset than to Daven with a Minyan after sunset? Some Poskim rule that it is better to Daven Mincha in private before sunset than with a Minyan after sunset. Other Poskim however rule it is better to Daven Mincha with a Minyan after sunset, then to Daven in private beforehand. It is unclear from Admur as to what his opinion is on the matter.

May one Daven Mincha past Tzeis Hakochavim? No. 

May one Daven Mincha and Maariv within Plag Hamincha on Erev Shabbos or during the week?[19] It is forbidden to Daven both Mincha and Maariv on a single day within the period of Plag Hamincha.  Some Poskim, however, are lenient to allow a congregation to Daven both prayers one after the other within the period of Plag Hamincha, if it will be difficult to gather the congregation again afterwards for Maariv. Other Poskim, however, rule one may only be lenient to do so if one will Daven Maariv after sunset. There are communities that follow the former opinion and Daven both Mincha and Maariv within Plag Hamincha. It is never permitted for an individual that is not part of a Minyan to Daven both Mincha and Maariv within Plag Hamincha, as stated above.

The Zeman for Maariv:[20]

See Chapter 37 Halacha 1-2 for further details on this subject!

1. The start time:

Start time for nighttime Shema: The time for the nighttime Shema begins from when three small stars become apparent in the sky. At this point nighttime has definitely arrived. On a cloudy day in which it is not possible to tell how many stars are out, then one should wait until he is certain that my time has arrived. In the event the one read the Shema prior to nightfall, but after the time of Plag Hamincha, such as if one Davened Maariv prior to nightfall, then one is to repeat its reading after nightfall without its blessings. One is to repeat all three paragraphs of the Shema after nightfall.

Start time for Maariv-The custom-Tzeis Hakochavim: The start time for the prayer of Maariv is from Tzeis Hakochavim. [Although from the letter of the law it is permitted to Daven Maariv starting from Plag Hamincha, and so was done in previous times, nonetheless today the widespread custom is not to Daven Maariv until after Tzeis Hakochavim.]

The letter of the law-Plag Hamincha: From the letter of the law, it is permitted to Daven Maariv starting from Plag Hamincha. The above applies only if one Davened Mincha prior to Plag Hamincha. However, if one Davened Mincha pass the time of Plag Hamincha, then it is forbidden from the letter of the law to Daven Maariv prior to Tzeis Hakochavim, unless one is with a Minyan and it is a time of need such as on a fast day, as explained above.

Repeating Shema after nightfall: In the event that one Davened Maariv prior to Tzeis Hakochavim, then he must remember to repeat all three paragraphs of Shema after nightfall.

With Minyan after Pelag: One who is accustomed to always Daven Maariv after nightfall is not required to Daven beforehand with the Minyan even if this will cause him to not be able to Daven with a Minyan later on.

On a fast day: On a fast day, one may begin Maariv after sunset, prior to Tzeis Hakochavim, in order to conclude Maariv and eat at the earliest possible time.

 

The end time:[21]

Shema-Chatzos/Alos: Initially, one is to read the Shema immediately upon nightfall and so is considered the greatest way of fulfilling the mitzvah. At the very least, one is to say the Shema prior to midnight due to a Rabbinical decree against pushing it off until morning, lest one forgot to say it. Nonetheless, Bedieved, if midnight has already passed and he has yet to read it, then he is to read it until morning.

Maariv-Alos: It is permitted for one to Daven Maariv up until Alos Hashachar, although initially one is required to recite the paragraph of Shema prior to midnight, as stated above in the previous Halacha. After Alos Hashachar has arrived one may no longer recite the Shemoneh Esrei prayer of Maariv, and one is able to make it up as a Tashlumin prayer after Shacharis. Many Poskim[22] rule that even if he delayed its prayer due to reasons beyond his control, in which case we rule that one can make up the section of Shema and its blessings after Alos, as will be explained in Chapter 37, nevertheless, he cannot Daven Shemoneh Esrei. [Thus, even one who did not yet Daven Maariv due to reasons beyond his control and Alos has already arrived, he is to only Daven Maariv until the blessing of Goal Yisrael. He does not recite the blessing of Hashkiveinu, nor does he Daven Shemoneh Esrei of Maariv. Rather, in the morning after Shacharis he should Daven Shemoneh Esrei again as a Tashlumin for Maariv.]

Reading both the evening and morning Shema after daybreak prior to Mi Sheyakir or sunrise: In the event that due to reasons beyond one’s control one was unable to read the evening Shema at night, then he is to read it after daybreak, up until sunrise. In such a case, he may not also read the morning Shema after daybreak even if he will be unable to read it later on after the time arrives. Even Bedieved that one read both the night and morning Shema between daybreak and Mi Sheyakir, or daybreak and sunrise according to the latter opinion, he must reread the morning Shema after sunrise. Thus, a person should initially be stringent that if he had to say night Shema after daybreak than on that day he should not daven or recite the morning Shema prior to sunrise, and thus even if he is accustomed to Davening Vasikin, he should not do so on that day. Nonetheless, in a time of need, he may be lenient to do so after the time of Mi Sheyakir.

The Zeman for Musaf:[23]

7. Start time:

After Shacharis: The time of the Musaf prayer begins immediately after Shacharis. Even if one Davened Shacharis very early in the day, immediately after its time of arrival, nevertheless one may also Daven Musaf at that time.

Davened before Shacharis: If one transgressed and Davened Musaf before Shacharis he nevertheless fulfills his obligation. However, initially, one may never Daven Musaf before Shacharis even if this will cause him to miss Davening Musaf with a Minyan, such as he arrived to Shul when the congregation is Davening Musaf. In such a case he is to first Daven Shacharis and then Daven Musaf privately.

1. The end time:

The 7th hour of the day: One may not delay the Musaf prayer past seven [Zemaniyos] hours into the day, which is one hour after midday. One who delays Musaf past this time is called a Posheia, one who is negligent.

Bedieved: Nonetheless, one fulfills his obligation even if he prayed Musaf past this time being that the prayer may be recited throughout the day. 

If it is already past sunset, but before nightfall, may one still Daven Musaf? Yes.

Forgot to Daven Musaf until night-Tashlumin:[24] If one forgot to Daven Musaf throughout the entire day, until nightfall, he cannot make up the prayer with a Tashlumin Shemoneh Esrei as is usually done for other prayers.

1. Mincha versus Musaf:[25]

After Mincha: Even if one already Davened Mincha he may nevertheless still Daven Musaf.

Which is one to Daven first-Letter of law: If one delayed Davening Musaf until the time of Mincha arrived, which is from 6.5 hours into the day, and hence now has two prayer obligations in front of him, one of Musaf and one of Mincha, then if it is past the time of Mincha Ketana he is required to first Daven Mincha. If, however, it is prior to Mincha Ketana and he does not desire to currently eat a meal, then it is disputed as to whether he must precede Mincha or if he may choose to precede Musaf. Practically, it is proper to suspect for the stringent opinion and to therefore always be careful to Daven Musaf before the time of Mincha arrives, which is 6.5 hours into the day.

Which is one to Daven first-Kabbalah and custom: The above law applies according to the revealed aspects of Torah. However, according to Kabala, one is always to precede the prayer of Musaf to that of Mincha, even if it is past Mincha Ketana. Practically, the widespread custom today is to always precede Musaf to Mincha even if it is past Mincha Ketana and one desires to eat a meal. This certainly applies if one does not desire to Daven Mincha at this time but rather later on. If, however, one now has a Minyan available for Mincha, then he is to Daven Mincha with the congregation and only afterwards Daven Musaf.

By a Minyan: The entire law above only applies to one who is Davening in private. If, however, a Minyan is Davening and delayed the prayer of Musaf until 6.5 hours into the day, or even past Mincha Ketana, they are not to precede the prayer of Mincha to Musaf. 

Mincha versus Musaf-No time to Daven both: If one does not have time to Daven both Musaf and Mincha, such as it is already very close to nighttime, then it is disputed as to whether he is to Daven Musaf or Mincha, and practically, Admur rules like the latter opinion that one is to Daven Mincha in such a case.

The Zeman for Hallel:[26]

The start time: Hallel may be recited starting from dawn [Alos Hashachar]. Nevertheless, Lechatchila one should not recite Hallel prior to sunrise. However, if one transgressed and recited Hallel prior to sunrise, after daybreak he has fulfilled his obligation.

After Shemoneh Esrei: Although Hallel may be recited throughout the day, it is best to recite it immediately after the Shacharis Shemoneh Esrei. It is permitted to recite Hallel prior to Davening Shacharis. Nevertheless, according to the Arizal, Hallel is not to be said prior to Davening.

The end time: Hallel may be recited throughout the entire day until nightfall [Tzeis Hakochavim]. Thus, if one did not recite Hallel immediately after Davening [Shemoneh Esrei of] Shacharis, he may recite it throughout the day, until Tzeis Hakochavim.

May Hallel be recited with a blessing after sunset, prior to nightfall? No. One is to recite Hallel without a blessing.

May one eat before Hallel? One is to avoid eating prior to reciting Hallel, unless he is sick or weak in which case he is not to be stringent even as an act of piety. [Thus, even if one already Davened Shacharis but has not yet recited Hallel, he should avoid eating until he recites Hallel.]

  The Zemanim-Calculating the Zemanim:

9. When is Alos Hashachar?[27]

There are various calculations and opinions for when the time of Alos Hashachar begins. The opinions vary between 72 minutes prior to sunrise, 90 minutes prior to sunrise, 96 minutes prior to sunrise, 120 minutes prior to sunrise. Practically, one is to be stringent like all opinions.  Thus, regarding a fast and Sefiras Haomer, one is to be stringent to consider Alos Hashachar to begin approximately two fluctuating hours prior to sunrise. Regarding morning blessings, Birchas Hatorah, Shema, Tefila and the like, one is to be stringent to calculate Alos Hashachar 72 fluctuating minutes before sunrise.

1. When is Mi-Sheyakir?[28]

The exact time of Mi-Sheyakir is difficult to ascertain, and there are various opinions in its regard. Practically, the widespread custom in Jerusalem/Eretz Yisrael is to consider Mi Sheyakir exactly 60 minutes before sunrise throughout all times of the year, although other customs do exist. The widespread custom in the Diaspora is to consider Mi Sheyakir either from when the sun is 12.9 Degrees from the horizon, or 11.5 degrees from the horizon. Practically, each person is to follow a calendar that contains the listing of his city, and follows the Halachic opinion that he is accustomed to follow. There is no known Chabad custom in this regard.

1. When is Neitz-sunrise:[29]

There exists a 2-4 minute difference between the astronomic sunrise which is when the sun reaches above the horizon, and the visible sunrise, which is when the sun reaches above the highest altitude in the area. Practically, according to most Poskim, we follow the time of visible sunrise and not the astronomic sunrise, and so is the widespread custom.

1. When is Sof Zman Kerias Shema?

The dispute: There is a difference of opinion as to whether the end of Shema hours are calculated from daybreak or from sunrise. The former approach is commonly known, and published, as the time of the Magen Avraham for the end time of prayer. The latter approach is commonly known and published as the time of the Baal Hatanya and Gr”a.

The practical ruling: Practically, much of Jewry is initially stringent to recite the Shema which is Biblical within the earlier timeframe [i.e. Zman Magen Avraham].

Chabad custom: The Chabad custom follows the ruling of Admur in his Siddur, which extends the saying of Shema and prayer to the later timeframe as rules the second opinion. Nonetheless, even those who follow the Poskim who hold of the later time frame are to be careful to Daven at least 45 minutes prior to its end time.  [Thus, in conclusion, even according to the Chabad custom to follow the later timeframe [i.e. Zman Baal Hatanya], in actuality it is actually to be said close to, and at times even earlier, than the early time frame [i.e. Zman Magen Avraham] in order to say it at least 45 minutes before the end time.]

1. When is Sof Zman Tefila?[30]

The same debate relevant to the end time of reading the Shema applies likewise regarding the end time of prayer, with some calculating the four hours starting from daybreak and others calculating it starting from sunrise. The former approach is commonly known, and published, as the time of the Magen Avraham for the end time of prayer. The latter approach is commonly known, and published as the time of the Baal Hatanya and Gr”a. Practically, while much of Jewry is initially stringent to recite the Shema which is Biblical within the earlier timeframe [i.e. Zman Magen Avraham], as explained there, regarding the end time of prayer which is only Rabbinical they are lenient for any justifiable reason, such as to Daven with a Minyan.

Chabad custom: The Chabad custom follows the ruling of Admur in his Siddur, which extends the saying of Shema and prayer to the later timeframe as rules the second opinion. Nonetheless, even those who follow the Poskim who hold of the later time frame are to be careful to Daven at least 45 minutes prior to its end time. [Thus, in conclusion, even according to the Chabad custom to follow the later timeframe [i.e. Zman Baal Hatanya], in actuality it is actually to be said close to, and at times even earlier, than the early time frame [i.e. Zman Magen Avraham] in order to say it at least 45 minutes before the end time.]

_______________________________________________________________________

[1] See Admur 58 and 89; Siddur Admur; Kaf Hachaim and Piskeiy Teshuvos ibid and ibid; Ketzos Hashulchan 19-20; Tefila Kehilchasa chapter 3; Isheiy Yisrael Chapters 13, 18, 27, 28

[2] See Admur 58:1-11; Rebbe Eliezer Brachos 21a; Ketzos Hashulchan 19:11-13; Piskeiy Teshuvos 58:10

[3] See Admur 58:2; Ketzos Hashulchan ibid

[4] See Admur 58:6-8

Regarding reading both the evening and morning Shema after daybreak prior to Mi Sheyakir or sunrise: See Admur 58:9; Kuntrus Achron 58:1; Minchas Kohen 2:11; Kaf Hachaim 58:21; Piskeiy Teshuvos 58:11  

[5] See Admur 58:4; Michaber 58:1; Ketzos Hashulchan 19:12; Piskeiy Teshuvos 58:5  

[6] See Admur 58:5 and 46:9; Michaber 58:2; Rama 46:9; Rabbeinu Yonah Brachos 9b

[7] See Admur 46:9 Ketzos Hashulchan 14:1

[8] See Admur 58:1 and 3 and in his Siddur; M”A 58:1; Ketzos Hashulchan 19:11

[9] See Admur 58:10

[10] See Admur 58:10; Michaber 58:6; Tzemach Tzedek Brachos 1:2; Ketzos Hashulchan 19:13  footnote 34; Piskeiy Teshuvos 58:12

[11] See Admur 58:11; Piskeiy Teshuvos 58:13

[12] See Admur 89:1-2; Michaber 89:1; Rambam Tefila 3:1; Mishneh Brachos 4:1; Ketzos Hashulchan 20:1; Piskeiy Teshuvos 89:2-5

[13] See Admur 58:5; 89:1-2; Michaber 89:1; Rambam Tefila 3:1; Rebbe Yehuda in Mishneh Brachos 4:1; M”B 58:5; Aruch Hashulchan 58:20; Ketzos Hashulchan 20:1; Piskeiy Teshuvos 58:8; 89:9-10

[14] See Admur 80:3; 89:2; Rama 89:1; Perisha 89:2; Rambam Tefila 4:15-16 that prayer without concentration is completely nullified; Peri Chadash 89; Mamar Mordechai 89; Sefer Habatim on Rambam Tefila 3; Mishmeres Shalom Kudinav 9 footnote 1; 10:1; Maor Vashemesh End of Parshas Nitzavim; Sefer Sur Meira from the Bnei Yissachar 36; Derech Pikudecha Mitzvas L.S. 16; Eretz Tzevi 36; Biur Halacha 89; Igros Kodesh 3:147; 5:312; 6:354; 9:142; 13:235; 14:407; Sichas 19th Kisleiv 5711:25, printed in Toras Menachem Hisvadyus 5711 1:134; See also Sichas 20^th Menachem Av 5715 20, printed in Toras Menachem 14 p. 261; Likkutei Sichos 34:318; Sefer Hasichos 5748 2:670-671; Piskeiy Teshuvos 89:7; Article of Rav Yechezkla Feigin, printed in Hatamim vol. 8 p. 401; Article of Rav Avraham Heller, printed in Koveitz Chidushei Torah Mibeis Chayeinu 5760 p. 28; Sefer Zichorn Tov Inyanei Tefila 10; Mivaser Tov 2:2; Sefer Ben Yechiel on Tana Dvei Eliyahu p. 46; Divreiy Yitzchak 47; Tosafus Yesheinim Yuma 28b; Piskeiy Teshuvos 89:7

[15] See Admur 91:11; Michaber 90:10; Tur 620 in name of Rosh; Talmidei Rabbeinu Yona on Brachos 28b; Kol Bo 11; M”A 90:21 based on implication of Talmidei Rabbeinu Yona ibid; Bach 90; Elya Raba 90:11; Implication of Beis Yosef 90; Olas Tamid 90:11; Levush 90; Shisilei Zeisim 90:32; Kaf Hachaim 90:73; M”B 46:32; 90:36; Darkei Chaim Veshalom 145 that the Munkatcher would Daven alone when he saw that the congregation would pray after the time; Piskeiy Teshuvos 89:6

[16] See Michaber 233:1; M”A 233:2; Taz 233:1; Radbaz 676; Birkeiy Yosef 233:4; Shaareiy Teshuvah 233:1; Maaseh Rav 65 of Gr”a; Or Tzadikim 25; Ashel Avraham Butchach 233; Ben Ish Chaiy Vayakhel 3; Kaf Hachaim 233:1 and 3; Divrei Yatziv 1:98; Ketzos Hashulchan 26:1; Piskeiy Teshuvos 233:2

[17] Shraga Hameir 7:20; Piskeiy Teshuvos 233:1

[18] See Michaber and Rama 233:1; Mishneh Brachos 26a; Rambam Tefila 3:4; Talmidei Rabbeinu Yona on Rif Brachos ibid; Shiltei Hagiborim on Mordechai; Kneses Hagedola 233; Lechem Chamudos; Levush 233; Shaagas Aryeh 17; P”M 89 M”Z 1; 106 M”Z end of siman; P”M 233 A”A 7; Gr”a 233; Gr”a 261; Maaseh Rav 64 Ashel Avraham of Butchach 233; Aruch Hashulchan 233:8; M”B 233:7 and 14; Shaar Hatziyon 233:18; Maharam Shick 91; Rav Poalim 2:19 and Ben Ish Chaiy Vayakhel 7; Kaf Hachaim 233:4-5; Eretz Tzevi 1:1; Shut Beis Avi; Minchas Elazar 1:13 and Nimukei Orach Chaim 131:2; Meumei Sadeh on Brachos ibid [of Nesiv]; Divrei Yatziv 98-99; Or Letziyon 1:20; Yechaveh Daas 5:22; Mishneh Halachos 7:32; Piskeiy Teshuvos 233:6; Halacha Berura [Rav Dovid Yosef] 233 for list of many Rishonim and Achronim; Biurim on Seder Hachnasas Shabbbos [Levin] p. 65-67; Halacha Berura [Rav Dovid Yosef] 233

[19] See Admur 267:2; M”A 267:1; M”A 233:6; M”B 233:11; M”B 267:3; Biur Halacha 267:2 “Umiplag”; Poskim in Kaf Hachaim 233:12; Shraga Hameir 5:21; Kinyan Torah 4:27; Piskeiy Teshuvos 267:2 footnote 19

[20] See Michaber and Rama 233:1; Mishneh Brachos 26a; Admur in Shut 7; 482:2; Admur 267 Kuntrus Achron 1; Ketzos Hashulchan 27:1, 3-4

[21] See Michaber 235:4; Admur 58:9; Ketzos Hashulchan 27:1, 3-4

[22] M”B 235:34; Rambam Tefila 3:6; Kaf Hachaim 235:25; See Shaar Hatziyon 235:42

Other opinions: Aruch Hashulchan 235:19; Hisorerus Teshuvah 1:113 [Daven Nedava]; See Maharil Diskin Kuntrus Achron 2:5-29; Piskeiy Teshuovs ibid

[23] Admur 286:1-2 and 286:5; 620:1 Ketzos Hashulchan 83:5

[24] Admur 286:2 and 108:10; Michaber 108:6 and 286:1; Tosafus Brachos 26a; ; Ketzos Hashulchan 83:5

[25] Admur 286:5; Ketzos Hashulchan 83:6

[26] Admur 488:3; Mishneh Megillah 20b

[27] See Shiureiy Tziyon 37; Yagdil Torah Tzemach Tzedek 23 p. 23; Siddur Raskin p. 625 and Miluim 27; Rav Sangwai in Habracha 5:162; Piskeiy Teshuvos 89:2

[28] See Kaf Hachaim 18:18; Igros Moshe 4:6; Piskeiy Teshuvos 18:5

[29] See Piskeiy Teshuvos 58:7

[30] Daas Torah 56:6; Kaf Hachaim 233:7; Piskeiy Teshuvos 58:2; 89:6; Igros Kodesh 10:294; Likkutei Sichos 29:378