The following formula was given to me earlier on this forum and it works
great...thanks.

=SUMPRODUCT(--($A$1:$A4<A4),--($A$1:$A4>=A4-365),$B$1:$B4)

Now I need to understand how it works.  What is the signfigance of the --?

The formula is summing data in column B for dates found in colun A for the
previous 365 days from the date found in cell A4.  I thought that sumproduct
only multiplied the sums of arrays.  How does this work?

Thanks in advance.