There are various ways to represent the duration of time.
I gave you the expression I did because of this statement: "I want to populate a 3rd field (InvoiceTime) with the minutes between the 2 timedate fields…"
I took this literally to mean you wanted the duration represented in minutes.
If you want just the straight Duration (the equivalent of DateDiff()), then you can define the column as type “Duration” and use this expression:
[End] - [Start]
This will give you a result like I implied above “001:23:32” - which is one hour, twenty-three minutes and thirty-two seconds.
You said you Invoice by the hour so if instead it is more useful to have your result in fractional hours you could amend the expression I previously gave you and divide by 60 to put the result into hours like so:
(HOUR([End] - [Start]) * 60 + MINUTE([End] - [Start])) / 60
The duration of “001:23:32” would then be represented as 1.38 hours - a Decimal data type
It all depends on what you need.