Assuming what you are trying to achieve from the original two date/time values is a total number of hours (and minutes?) worked then you need to use something that doesn't return a date/time value, but a number.
To get the total time from the two date/time values use the DateDiff function using the "n" parameter to return the number of minutes and then subsequently divide by 60 to get hours and minutes.
To get just the Hours use integer division by 60 on the minutes result from DateDiff - the '\' operator , and for the minutes the modulo operator . Eg, minutes result from DateDiff MOD 60.
To get just the hours use the "h" parameter with DateDiff.
Always depend on the granularity to which you wish to store the hours. If it's just hours the then the Total, Personal and net times can be stored a Integer or Long data types, or if its for totals with fractional hours either store the minutes as long etc and manipulate later, or store as doubles having divided by 60 to get the fractional part which is the minutes.