I have managed to get the attached workbook working with your text
data.

I have modified the first 3 columns, to all show Month1, with
differing formulae, to prove that all 3 results are the same.

Column G has your original formula, looking at extra columns where you
have converted Text to Numbers

Column H, calculates the result using Sumproduct formulae. I am not
suggesting using this as a solution, because Sumproduct is much slower
than Sumifs.

Column F uses my revised formula

=SUMIFS(Amount,Account,"--"&$C6-1,Account,"<=--"&$D6,Dep,$B6,Month,F
$2)



This is using all of the original Text data.

I have coerced the Text to Numeric, using the double unary minus --


Roger Govier was good enough to send me this solution offline. I had
never seen the unary function before. Many thanks Roger.

It works exactly as I needed it to work.

Siegfried