In AD-Din Solution we allow organizations to select the calendar of choice for Islamic date on the Ad-Din prayer time widget. Both are prevalent. we recommend you do your due diligence. The default is Umm al-Qura.
For several decades Saudi Arabia has employed a calculated lunar calendar which is commonly referred to as the “Umm al-Qura calendar”. Many Muslim communities in non-Islamic countries also tend to follow the Umm al-Qura calendar as its use in mosques funded by Saudi Arabia and in modern computer software (it is the default Islamic calendar in the Arabic setting of Microsoft software) becomes more and more prevalent.
The Islamic (hijrī) calendar is used by more than a billion Muslims around the world to determine the main days of observance in the Islamic religious year. Although in daily life the Western (Gregorian) calendar is now generally followed, the Islamic lunar calendar has since the days of the prophet Muḥammad regulated the key days in the Islamic year such as the start of the month of fasting (Ramaḍān), the Breaking of the Fast (ʿĪd al-Fitr) and the Day of Sacrifice (ʿĪd al-Adḥā on 10 Dhu ʾl-Ḥijja) during the annual pilgrimage (Ḥajj) at Mecca.
Here is one edge case where you may end up switching the calendar in Ad-Din Solutions.
The committee you follow has declared that Rabi Ul Awwal will complete a 30-day cycle:
By default, the Umm al-Qura calendar may or may not have this accounted for in their calculation. Since the date correction is not supported in the Umm al-Qura calendar setting up 30th Rabi Ul Awwal may be a challenge.
While in Hijri Calendar you may be able to achieve this by adjusting the offset to either -1 or -2 to achieve 30th Rabi Ul Awwal.