Guessing ...
=SUMPRODUCT(--(A1:A100>=StartDate),--(A1:A100<=EndDate)*(B1:B100))
A1:A100 contain dates
B1:B100 contain numeric data
StartDate & EndDate are your two date parameters. These could be cells
containing required dates e.g. X1 & X2
=SUMPRODUCT(--(A1:A100>=X1),--(A1:A100<=X2)*(B1:B100))
Note SUMPRODUCT does not allow full columns i.e A:A is invalid, and arrays
mut be same size.
HTH