SUMIF can only use one criteria, so you will need to replace this part
of your formula with SUMPRODUCT, along the lines of:

SUMPRODUCT(('Sent to Assembly'!A1:A1000=A3)*('Sent to Assembly'!
D1:D1000=D3)*('Sent to Assembly'!B1:B1000))

I've assumed dates are in column D on both worksheets. I've also
assumed you have up to 1000 rows, as you can't use full column
references in SP (prior to XL2007). Change the cell references to
suit.

Hope this helps.

Pete

On Mar 12, 1:16*am, Richard wrote:
=IF(B3="","",MAX(B3-SUMIF('Sent to Assembly'!A:A,A3,'Sent to Assembly'!B:B),))
Is there any way to change this formula to when there is a date next to it
(columnD) it will only count those date that are the same, and so on.
Example: All dates that are the same do calculation for that date