Are these the same two bytes we saved by dropping the century from dates back in the Y2K days?  I'm going to argue for the dashes. The design principle that you want to store data the way it is used and use it the way it's stored. When you see 2019-03-19, you know that it's a date. When you see 252-77-6688, you know that it's a Social Security number. Likewise, 23:00:00 Hrs is clearly a time. The cost of a couple of pieces of punctuation is negligible these days, but the cost of an error in misreading the data is not negligible.