I'm trying to do a semi-advanced SUM which checks a sheet of timesheet data and if the day matches, sums the data for that user (Date is in Cell A, Username M and number of minutes in N).

My formula is as so:
=SUM(IF(Timesheets!$A$2:$A$1000=A2,IF(C$1=Timesheets!$M$2:$M$1000,Timesheets!$N$2:$N$1000)))
What's interesting is when I check this in the function arguments checker, it shows the correct value for that user/day combination but in the cell it's showing 0.

I've checked it's not a formatting issue (it's not) and all the references are right but I can't fathom it out. Any ideas?

Thanks in advance

Tim