Zeman Birchas Kerias Shema:[1]

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 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[2] 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[3], however, rule that one may read the Shema with its blessings anytime of the day up until sunset. Other Poskim[4] 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. [Thus, we find a major stringency regarding the section of Shacharis that includes the Shema and its blessings versus even the section of Shemoneh Esrei, as Shemoneh Esrei is permitted to be prayed up until midday in the event that one has delayed reciting it past the fourth hour, while Birchas Shema may not be recited after the fourth hour and is seemingly a blessing in vain according to the main opinion.[5] Thus, in addition to needing to be careful to finish Davening Shemoneh Esrei prior to the fourth hour of the day in order to Daven within Zeman Tefila, one must be even more careful regarding reading the Shema with its blessings prior to the fourth hour of the day, as past that time we rule that it may not be recited. Nonetheless, practically, 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.[6]]

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

[2] Admur ibid; Michaber 58:6

[3] Opinion in Admur ibid in parentheses; Rambam Shema 1:13; Rashba 192; Peri Chadash 58:6; Ketzos Hashulchan 19 footnote 34

[4] Tzemach Tzedek Brachos 1:2; Ketzos Hashulchan 19:13 and footnote 34, based on Mishkanos Yaakov; M”B; See Piskeiy Teshuvos 58:12 footnote 94 for all the opinions on this subject

[5] However, if one were to follow the second approach which permits its recital throughout the day, then we would find a leniency associated with the Shema with its blessings versus the prayer of Shemoneh Esrei of Shacharis of which there are no opinions which permit it past midday. Indeed, this opinion lends some justification for those who Daven Shacharis past midday, at least in regards to the blessings of the Shema and not have to be considered a complete blessing in vain. See Ketzos Hashulchan 19 footnote 34 who writes that many hold that it may be said until sunset

[6] See Piskeiy Teshuvos 58:12; Furthermore, regarding those who say it even past midday, one can argue that they rely on the dissenting opinion brought above. Accordingly, just as those who say it past the fourth hour of the day base their claim on the fact that the custom is like this opinion which allows its recital until the day, so too in those communities that it is accustomed to permit extending Shacharis to even past midday can claim that the custom is like the opinion that permits it until sunset. This is not God forbid to come to condone such behavior of Davening Shacharis past midday but simply is a Limud Zechus for those who do so.