Today is still April 18 in the US. It is the evening of the 18th in Italy. Sunset, local time, in Mecca was at 6:41 PM. That began the next day according to the most ancient traditions in the area. The "next day" is, in the Western calendar, the 19th.
This is probably another Islamist act. The Number 19 played a prominent role.
Whether it means anything to us, it sure seems to mean something to them, eh?!
April 19th is the date of the OKC Bombing, Battle of Lexington and Concord, Hitler's Birthday..take your pick