The NaT isn't the problem. You cannot concatenate datetime and double in a homogenous matrix:
>> x = [1 2 datetime('now')]
Error using datetime/horzcat (line 1387)
All inputs must be datetimes or date/time character vectors or date/time strings.
Either store the data in a table/timetable, which is intended to store variables of different types but the same height, or decide how you want to convert the datetime into something that can be represented as a double (i.e. if all you needed was hour-of-the-day, you could extract that), although that loses the datetime functionality.