Is using a dating and calender system based on Christianity , violating church and state ?

.
The world all uses a dating system based on Christianity, using Jesus’s birth as the starting date.

How is that not a violation of Church and State ?

Note: Im not against it, im just wondering why those others, who want to stop all religion from schools, government , etc.

Have never tried to sue over the dating system.

Answer
Why would you give those loons any ideas.