Create a query into your table.
In the query, enter this into a fresh column of the Field row:
Minutes: DateDiff("n", [start time], [end time])
Run the query and check that it is returning the correct number of minutes.
If not, chances are that you are entering minutes and seconds instead of
hours and minutes. If that is the case, this calculated fields would show
the number of seconds:
Seconds: DateDiff("s", [start time], [end time])
If the data has been entered incorrectly, remove the input mask and have the
users enter the information correctly in future. To fix the existing data,
you could run an Update query (Update on Query menu), and update the [end
time] field to:
DateAdd("n", DateDiff("s", [start time], [end time]), [start time])
Once you have the Minutes calculated correctly in your query, you can
display this on a form with a text box that has a Control Source of:
=[Minutes] \ 60 & Format([Minutes] Mod 60, "\:00")
It's generally best to do your calculations in minutes (or seconds) in
Access, and then display the result however you like. This solves the
problem of summed times wrapping into weird dates once they get past 24
hours.
In general, durations are best stored as Number fields (in whole minutes (or
seconds)) rather than Date/Time fields.
