Until when during the day may one put on Tefillin?
One is not to put on Tefillin after the start of sunset [i.e. Bein Hashmashos].
If one did not put on Tefillin the entire day, may he do so after sunset, before Tzeis Hakochavim?
If one did not wear Tefillin the entire day, and it is now after sunset, he is to put on Tefillin so long as it is before nightfall [i.e. Tzeis Hakochavim]. When wearing Tefillin after sunset, the Tefillin is to be worn without a blessing.
May one put on Tefillin by Mivtzaim after sunset?
May one wear Tefillin past Tzeis Hakochavim of the Geonim, but prior to Tzeis Hakochavim of Rabbeinu Tam?
May one wear Tefillin of Rabbeinu Tam past Tzeis Hakochavim of the Geonim, but prior to Tzeis Hakochavim of Rabbeinu Tam?
If one is already wearing Tefillin may he continue to wear them past nightfall?
Past Sunset: If one was wearing Tefillin prior to sunset he does not need to remove the Tefillin when sunset arrives.
Past Tzeis Hakochavim: If one was wearing Tefillin prior to nightfall [i.e. Tzeis Hakochavim], when nightfall arrives he is to remove his Tefillin if he is in a public area. If however one is in private, in an area without any other people, he does not need to remove the Tefillin even after nightfall. Likewise, if one is in a Beis Midrash, even if there are other people around, he may continue wearing them after nightfall.
Wearing past Tzeis Hakochavim in order to guard the Tefillin: If one did not remove his Tefillin prior to Tzeis Hakochavim because he did not have an area to guard them [from being stolen or vandelized, and he is thus wearing them in order to guard them, then it is permitted to remain wearing them, and so is the practical directive. (Nevertheless, one is required to tell the onlookers that he is doing so in order to guard the Tefillin.) It is however forbidden to initially put on the Tefillin at night in order to guard them. (In addition, the initial directive given is that one should not place the Tefillin on even before nighttime in order to wear them into the night and guard them, even though this is permitted from the letter of the law.)
Erev Shabbos and Erev Yom Tov: On Erev Shabbos and Erev Yom Tov one must remove the Tefillin prior to sunset, sometime prior to candlelighting on Erev Shabbos.
 Siddur Admur [letter 19 Raskin]; Implication of Beis Yosef 30; opinion in Pâ€M 30 Aâ€A 2; Mâ€B 30/3
Ruling of Admur in Shulchan Aruch and other Poskim:Â In the Shulchan Aruch 30/2 [see also 18/8 regarding Tzitzis] Admur rules that one may don Tefillin until nightfall/Tzeis Hakochavim. So also rules the Mâ€A 30/2; Mordechai [brought in Beis Yosef ibid]. Some Poskim rule one may wear it even with a blessing until Tzeis Hakochavim. [Chayeh Adam 14/16; Artzos Hachaim 30; Shulchan Shlomo 30; Piskeiy Teshuvos 30/2 footnote 11-12] Other Poskim rule one is to wear it without a blessing. [Kaf Hachaim 30/17; Biur Halacha 30/5 â€œVeyeishâ€ concludes to wear the Tefillin without a blessing]
 The reason: Perhaps Admur in the Siddur follows the ruling of the Rambam that Biblically prohibits wearing Tefillin after sunset. [See Rambam 4/10; footnote 78 in Siddur Raskin] Alternatively, Admur in the Siddur suspects for the opinion of the Arizal who prohibits wearing Tefillin at night. [Piskeiy Teshuvos ibid]
 Pâ€M 30 Aâ€A 6; Mâ€B 30/2 and 5; Rav Ginzberg in Hiskashrus 889; Rav Rubinovitch in Hiskashrus 807, this is in contrast to the ruling suggested by Rav Raskin that after sunset it is forbidden.
Does this apply even according to Admur in the Siddur? Some write that the ruling of Admur in the Siddur does not apply to one who did not yet don Tefillin that day. [Rav Ginzberg in Hiskashrus 889; Author of Piskeiy Teshuvos brought in Hiskashrus 807] Others argue that perhaps according to Admur in the Siddur doing so is forbidden. [Rav Raskin in Miluim on Siddur]
 Mâ€B ibid
 Poskim of previous Question; This applies even in accordance to those who follow the ruling of Admur in the Siddur: Rav Ginzberg in Hiskashrus 783/787/800/807; Author of Piskeiy Teshuvos brought in Hiskashrus 807,
 Author of Piskeiy Teshuvos ibid
 As many Poskim rule Tefillin may be worn even initially until nightfall with a blessing, and so rules Admur in 30/2. Hence why be stringent on them like the ruling of the Siddur. [ibid]
 As today we are accustomed to consider Tzeis Hakochavim of the Geonim as Tzeis Hakochavim for all matters. However see Piskeiy Teshuvos 30/2 footnote 10
 Piskeiy Teshuvos 30/2
 The reason: As the two disputes are completely independent of each other, and we hold that after Tzeis Hakochavim of the Geonim it is nighttime.
 Siddur Admur [letter 19 Raskin]; Admur 30/1-2 and 4 implies that even initially one may put on the Tefillin after sunset, prior to nightfall, as brought in the previous in Halacha B!; Ateres Zekeinim 30/2
Other opinions: According to Kablala, one is to remove his Tefillin prior to sunset, and so was the custom of the Arizal. [Yad Ahron in Hagahos Tur; Birkeiy Yosef 30/2; Shalmei Tzibur p. 9; Siddur Beis Oved; Shaareiy Teshuvah 30/3; Yifei Laleiv 30/2; Kaf Hachaim 18/11-12; 30/5 and 7]
 Admur 30/2; Michaber 30/2 that Halacha Vein Morim Kein; see Menachos 36b
 Admur ibid; Mâ€A 30/3 in explanation of Michaber ibid
The reason: From the letter of the law, it is only forbidden to put on the Tefillin at night, however if one had them on prior to nightfall [Tzeis Hakochavim] he may continue wearing them past nightfall, and may remain wearing them for even the entire night. Nevertheless, practically we do not rule this way [Halacha Vein Morin Kein] due to worry that one may come to initially wear the Tefillin at night. Thus, one may not continue wearing them in the presence of other people so people donâ€™t come to think it is permitted to wear Tefillin at night. [Admur ibid]
 Admur ibid; Taz 30/6
The reason: As in a Beis Midrash the people are knowledgeable and aware that you cannot initially put Tefillin on at night, and hence they will not come to make a mistake. [Admur ibid]
 Admur ibid; Michaber ibid; Rebbe Eliezer Menachos 36b
A traveler wearing Tefillin past nightfall: One who is traveling [by foot] and does not desire to remove his Teifllin lest they fall, he may continue to wear them past nightfall, until he arrives home. Nevertheless, he is to cover the Tefillin.Â [Admur 30/4; Michaber 30/4; Beitza 15a]
 Admur ibid in parentheses; Mâ€A 30/4
 Admur ibid; Taz 30/4
 Siddur Admur [letter 19 Raskin]; Admur 30/4; Admur 29/1 â€œWhen removing the Tefillin on Erev Shabbos by Bein Hashmahos, when one is obligated to remove them]â€