John Joe wrote:What did 2016-12-13 23:59:59 and 2016-12-14 23:59:59 mean ?
Those are literal values which refer to a particular time on a particular date. The first one means 2016 December 13th at 59 minutes and 59 seconds past hour 23, which many people refer to as 11 PM. (If you're still a bit confused, the "12" is because December is the 12th month.) So you have "Year-Month-Day Hour:Minute:Second".
You should be able now to explain what the second one means, I hope.