Not just when the last third starts — see exactly how many hours and minutes you have for Tahajjud tonight, from the start of the last third until Fajr.
Time available for Tahajjud tonight
--h --m
Last third begins
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→
Fajr
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The night from Isha to next Fajr is split into three equal parts; the last third is the classically recommended Tahajjud window, and this is its total length.