Date fields are stored as double number datatypes no matter how they are displayed e.g 42165.25
The integer part is the number of days since 30Dec 1899 and the decimal part is the fraction of a day.
So 0.25 is 06:00 or 6.00am
Use this fact to calculate your time diff in hours
Convert datetime values to doubles, and calculate the difference, multiply by 24 to get hours.
Code:
=24*(CDbl([End])-CDbl([Start]))
This will give results like 12.75.
If you want this to be shown as 12:45 do a further calculation to get the minutes e.g. 60*0.75
If you only want the whole number of hours, either use Round function with 0 d.p. Or the Fix function to get the completed hours.
For an answer of 12.75, these will give 13 rounded or 12 using Fix