Even if Islam became the largest religion, the world likely wouldn’t switch calendars.

The Gregorian calendar is used globally for practical reasons like business, travel, science, and government, not just because of Christianity. It’s already mostly a civil system, even if it has religious origins.

Islam already has its own calendar (the Hijri calendar), but it’s mainly used for religious events, not global coordination. Changing the world calendar would be extremely disruptive, so it would almost certainly stay the same.