ttB = standardizeMissing (ttA, indicator) replaces every value matching indicator with the missing value of its own type, so that ismissing and rmmissing will afterwards treat it as missing.

'DataVariables' restricts it to the variables named. The row times are not a data variable and are never rewritten, so the time step is unchanged.

See also: ismissing, rmmissing, timetable

Source Code: timetable