Try:

=SUMIFS(G2:K2,$G$1:$K$1,"=" &M12,$G$1:$K$1,"<=" &M15)

"BillGr" wrote:

Where cells M12 and M15 contain the number 39315 and 39317, respectively.
The formula yields no results when M12 and M15 are in the formula. However,
when I substitute the actual number in the formula, the proper result is
produced.

=SUMIFS(G2:K2,$G$1:$K$1,"=M12",$G$1:$K$1,"<=M15") does not work
=SUMIFS(G2:K2,$G$1:$K$1,"=39315",$G$1:$K$1,"<=393 17") works