Why is this important?
As a Muslim, we have five obligatory prayers: at dawn (Fajr); at noon (Dhuhr); in mid-afternoon (‘Asr); at sunset (Maghrib) and at nightfall (‘Isha). There is limited time to pray Fajr as its time ends a little before sunrise. The Noon prayer can be prayed in the time before ‘Asr starts. ‘Asr has a few hours to pray in, depending on the time of year, but it is preferred to complete it as early as possible. Maghrib, I’m personally not sure about, but I was always told to pray it immediately without delay. All that said, ‘Isha seems to have the most relaxed schedule because the next prayer isn’t until dawn. I would love winter days because the sun would go down at 5pm, leaving the whole night available to do things and not worry about a prayer time intercepting. My habit is of praying ‘Isha right before sleeping so I felt like I had all the time in the world after Maghrib.
I remember always hearing that ‘Isha must be prayed before midnight. At some point however, after very surface-level conversation with people on the topic, I felt that the notion was either not authentic or didn’t make sense. The problem might have been that I was thinking all Gregorian.
Today hearing a scholar speak about this matter brought me back to the subject. He explained that on the lunar calendar, midnight does not occur at 12am all the time of course—it depends on the time of the year (which feels so obvious now).
The question originally asked of him was whether ‘Isha can be delayed until Tahajjud time. He said that without one of the two legitimate reasons—forgetting or accidentally falling asleep—no, it is not permissible to offer ‘Isha during the designated Tahajjud hours of the night. If one forgets or oversleeps past midnight, he must pray it as soon as he remembers. However, other than the two reasons mentioned, ‘Isha must be prayed by midnight.
We calculate Tahajjud time by dividing the hours between sunset and fajr into three parts. Similarly, we calculate midnight by dividing those same hours into two parts. So if Maghrib is at 6pm and Fajr at 4am, it equals a duration of 10 hours and the midpoint would be 11pm. This will change through the seasons though, shifting a little bit every day. So midnight may occur at 11pm, 12am, 1am—whichever the calculated hour may be—according to this scholar, lunar-midnight is the actual end time for ‘Isha salah.