Use Excel "serial dates" and format them to show time.

Excel uses a date scheme called serial dates, in which a day is an
integer number and a time is a fractional part of a day. Right now the
time is 12/22/2005 15:58 in my time zone; the serial date equivalent of
that is 38708.66530. Test this by entering =NOW() into any cell of
your spreadsheet, then format that cell to view as a number with a few
decimal places.

If you set up your dates using serial dates, your formula will work
properly.