I was helped yesterday with creating a complex formula using the SUMPRODUCT
function. It worked GREAT and I thank you VERY much. I mostly understand how
it works, but I did not understand the use of -- in it. Here is the formula:

=SUMPRODUCT(--(Dates!B$4:B$21=A13),--(B13=Dates!C$4:C$21),--(B13<=Dates!D$4:D$21),Dates!E$4:E$21)

Can you explain the formulas use of the "--"'s? Thanks!!