Is it necessary to rely on the 17-degree angle for the entry of the 'Isha prayer time, and if this angle is linked to the disappearance of the white twilight, is it permissible to change the calendar to rely on the disappearance of the red twilight to facilitate matters for Muslims, instead of combining the two prayers?

The time for the 'Isha prayer begins with the disappearance of the red twilight and extends until midnight. If the horizon is obscured by walls and mountains, it is preferable to wait until the white twilight disappears as a precaution. If performing the 'Isha prayer within its prescribed time becomes difficult, it is permissible to combine it with the Maghrib prayer by performing them together earlier (jam' taqdim). In this case, the Tarawih and Witr prayers are offered after 'Isha.