Clocks start showing 13:13 and everyone (except for people born in a leap year) goes crazy and starts killing themselves and each other. One of the characters figures out that if it weren’t for leap years, today would be 13/13/13. Seriously, that’s it. That’s the whole plot.
So, leaving out the really lame excuse for naming this 13/13/13 when they had run out of room on the calendar, this is a lot like The Signal (2007) except that was, you know, good. You should just go watch that instead.