I think fotis1991 is right. I was trying to find a way to avoid putting everything in one column because my employer wanted everything visually segregated by month. But unless I start building pivot tables, I think this is the best way to do it. Added a column for "month" to sort.

One remaining problem: using the first function

=SUMPRODUCT((CLIENT_ID<>"")/COUNTIF(CLIENT_ID,CLIENT_ID&""))

yields a #DIV/0 error when the range includes blank cells. I can't figure out why - shouldn't the ("CLIENT_ID<>"") part preclude blank cells from the function? I tried updating the function to

=SUMPRODUCT((CLIENT_ID<>"")/COUNTIF(CLIENT_ID,IF(CLIENT_ID=0,0,CLIENT_ID)))

but I'm still getting a zero error. Thoughts? Thanks guys!