time over 24hr not calculating

if (HasValue(startsamplingtime) and HasValue(endsamplingtime)) then

   var millisecondsPerMinute = 1000 * 60;

   var millisecondsPerHour = millisecondsPerMinute * 60;

   var interval = Time2Num(endsamplingtime.formattedValue, "HH:MM:SS") - Time2Num(startsamplingtime.formattedValue, "HH:MM:SS");

    // Calculate the hours, minutes, and seconds.

   var hours = Floor(interval / millisecondsPerHour );

   interval = interval - (hours * millisecondsPerHour );

   var minutes = Floor(interval / millisecondsPerMinute );

   interval = interval - (minutes * millisecondsPerMinute );

   var seconds = Floor(interval / 1000 );

   Concat( Format("Z9", hours), ":", Format("99",minutes), ":", Format("99", seconds))

endif

Need to incorporate date to make this calculation work right for over 24hrs. Don't want to have a date/time field if possible.  Would like to keep the fields separate.  For example:

Start Date 01/02/2014       Start Time 12:23:52  

End Date 01/03/2014        End Time   01:23:52

Total Time 1:00:00

Thanks