If the time  is a normal date/time value it will not matter. The datetime
value is a decimal number of days since some long distant date that is it as
two parts and integer part for the day and a decimal part for the fraction
of a day.

If you use this variable you can subtract the start time from the end time
without problems. ten past  midnight will have a higher integer value than
ten to midnight the day before. You only need to check that the call  was
not completed before it started  by checking end<start.

If you have reinvented the way to track time I suggest you rethink.