Expression Builder HELP

[Richard G]

I've built a query against a form and included a calculation to give me the average sum which looks like this :

=Sum([data field])/31*12.5

This work fine however, what is the best way to overcome the different number of days in the month ie; 28, 30 and 31? I've looked at the Expression Builder but getting lost

Regards

Richard

 

[Uncle Gizmo]

and 29!.......................

 

[RuralGuy]

Here's a formula for the last day of the current month:
Day(DateSerial(Year(Date), Month(Date) + 1, 0))