ISO 8601

An internet standard to write dates: YYYY-MM-DD where YYYY is the year, MM is the month and DD is the day.

It's also possible to add time in a compact way, but here I'll only show the "extended" format which is clearer: YYYY-MM-DD hh:mm:ss where hh is the hour (0-23) mm is the minute and ss the second. It's possible to drop both minutes and seconds.