What formula can I use to get the sum of cells in 1 column, but only if that
cell corresponds to a date in January. For example, column A is the date
column and every cell has a different date in it and I want to get the sum of
all the entries in column T in January or February etc.


The date column is setup in the format of dd/mm/yy

I've tried
=SUMPRODUCT(--(A4:A100000/1/7),--(A1:A10000<0/2/7),--(AP4:AP10000))

and I've tried =SUMPRODUCT((JANUARY(A4:A10000)=1)*(AP4:AP10000))

but neither has worked. Help!