This is a multi-step process and it is more complicated than you might think. Since you did not say which SmartSuite program you are using, I will just give you the "theory" here:
1) Determine the number of hours and minutes on the start day by subtracting the start time from midnight (24:00)
2) The number of hours and minutes on the end day is simply the time the employee stops working.
3) Determine whether more than one day has passed between the two date-time entries and add 24 hours for each day. You cannot simply use the day number because the employee may have worked across the end of a month or year. See if your product has a "DayOfYear" function and use it along with the year values in the two dates to determine the number of days elapsed and subtract 1. Then multiply the number of days by 24 to get hours.
4) Add the values from #1, #2 and #3 to get total elapsed hours and minutes.
Sue Sloan