it means you have until midnight Sunday 18th @ 12:00, which is sort of Monday Morning because it is a.m.
another words you still have until midnight, Eastern Standard Time or Sunday Night until 12
this confusion is why he posts the time you have remaining, just put 18 up there twice by mistake. (the Monday he meant was the 19th)
there are so many submissions from different parts of the world & not all observe Daylights Savings Time or the same times that we on the east coast observe it, so use the link.
another words, just use this countdown if you have any questions on the time remaining until the deadline:
http://www.timeanddate.com/countdown/to?iso=20150119T00&p0=207&msg=December+Game+Jam+Deadline+Countdown