TimeDuration represents time periods expressed in units of hours, minutes, seconds and milliseconds. Whilst we can't say how long a month is without knowing the year and the name of the month, we know how long a second is independent of the date.
This is not 100% true for minutes. Minutes can be 59, 60 or 61 seconds long (due to leap seconds.)
If you ask Duration to convert itself to milliseconds then it will work on the basis of 60 seconds in a minute. If you add or subtract it from a date it will take leap seconds into account.
@author John Wilson tug@wilson.co.uk