You are showing text. A timetable's row times are not text, they would be (in your case) datetimes.
retime is the way to synchronize a timetable to a regularly-spaced time vector.
You are asking about datenums, but again, a timetable uses datetimes. Not sure what you mean by "add to the end", but if tt is your timetable, and you've used the default name for the row times, you may be looking for
tt.Time = tt.Time + milliseconds(500);