Here is some information on Time in Excel.
Times in Excel appear difficult, but if you ensure that all data entry follows a time format that Excel recognizes, you will be able to add and subtract times without any need to use any special calculations.
Enter start and finish times as hh:mm, for example 07:30
Format all the cells used for time entry and time calculations with "hh:mm"
Use cell data entry validation to restrict entries to times (Toolbar - Data - Validation and select Time)
The duration of work is just Finish-Start
Start time in A2 07:30
Finish time in B2 11:10
Duration in C2 has the formula =B2-A2
and if formatted "hh:mm" will show 3:40, i.e., 3 hours and 40 minutes
If staff may work past midnight use the following formula
This adds 1 to the finish time when the finish time is 'earlier' than the start time, because the finish time is in the next day.
Excel's date/time serial number holds dates in the integer (whole number) part of the serial number and times in the decimal part of the number.
For example 09 September 2009 is 40066
12 noon on 09 September 2009 will be 40066.5 i.e. half way through the day.
If you need to calculate pay you will have to convert hours and minutes to fractions or percentage of a day, for daily rates or convert minutes to fractions of an hour for hourly rates.
Only do your decimal conversion for payroll calculation as a final step - don't mix decimal calculations and Excel's time serial numbers,
i.e., calculate time differences using Excel's time system, then do the conversion on the result, don't try and convert the start and finish times separately.