Reference: al-Muntaqaa min Fataawa Shaykh Saalih al-Fowzaan – Volume 1, Page 257, Fatwa No.153
Question: Is adopting the Anno Domini (A.D.) calendar considered as [displaying] loyalty to the Christians?
Response: It is not considered as [displaying] loyalty [to the Christians], rather, it is considered as resembling them.
And during the time of the Sahaabah (radhi-yAllaahu ‘anhum) the Anno Domini calendar was in existence, yet they did not use it. Rather, they desisted from using it, instead turning to the Hijree calendar. So they established the Hijree calendar and did not use the Anno Domini calendar, despite the existence of the Anno Domini calendar in their time.
This is evidence [enough] for the Muslims to be independent of the ways of the disbelievers and blindly following them, especially since the Anno Domini calendar is a symbol of their religion. This is because it is symbolic of the[ir] glorification of the birth of [Prophet] Jesus (‘alayhis-salaam) and celebrating it at the end of the year. And this is an innovation which the Christians have introduced; so we do not participate with them and nor do we encourage them upon this; And if we were to adopt their calendar, then that would mean we are resembling them, whilst we have the Hijree calendar – and al-Hamdu-Lillaah – which was introduced by the Leader of the Believers ‘Umar ibn al-Khattaab (radhi-yAllaahu ‘anhu) the rightly guided Khaleefah, in the presence of the Muhaajiroon and the Ansaar; [and] this is sufficient for us.