Hello Jeff,
I won't comment on the other options suggested by Alistair and Kevin, I don't have enough expertise to do so.
On the possible formula options I think it's almost impossible to accomplish with DSUM, even if you could do it the setup would probably be too complex to be worthwhile. SUMIFS will certainly be faster than SUMPRODUCT, the syntax for your example would be like this for B4 then copied across and down.
=SUMIFS(Import!$F$2:$F$2501,Import!$A$2:$A$2501,">="&$B$1,Import!$A$2:$A$2501,"<="&$D$1,Import!$O$2:$O$2501,$A4,Import!$I$2:$I$2501,"Billable",Import!$C$2:$C$2501,B$3)
regards, barry