One could argue that a date is a region in spacetime. It starts at the dateline, spreads eastward, and ends at the dateline some 48 hours later. (Edit: 50 hours is closer to the truth. And one could quibble about the definition of “dateline”, as Samoa moved across it once.)

Interesting take from 1999: Erik Naggum: The Long, Painful History of Time

http://naggum.no/lugm-time.html