Is it permissible to follow the Umm al-Qura calendar for the 'Isha prayer, such that it is prayed 90 minutes after Maghrib, or must one follow the North American calendar which determines the 'Isha time to be approximately 110 minutes after Maghrib, and is it permissible to combine the two calendars?

The fundamental principle for knowing the time of Isha is the disappearance of the red twilight (shafaq al-ahmar). If this is difficult to ascertain, there is no harm in relying on approved calendars, such as the Umm Al-Qura calendar, which scholars have attested to its accuracy and conformity with prayer times. There is no impediment to relying on it for the entry of Isha time. Sheikh Al-Uthaymeen stated that the Umm Al-Qura calendar does not bring forward the time of Isha. Therefore, it can be relied upon for praying with Muslims in the mosque. As for making the time interval between Maghrib and Isha in your country similar to that in Makkah, this is incorrect.