Time button

[DaveB]

I am trying to make a button that will take a field with
the time in it called CompletionDateTime and subtract it
from a field named StartDateTime and the program the
button so that when I click on it, it will enter the
figured time into a field called TotalTimeofBill. Can you
please tell me how to do this.

 

[Pavel Romashkin]

Like having this

Me.TotalTimeofBill = Me!CompletionDateTime - Me!StartDateTime

in the Click event of the button?
Pavel