Perfect! , Thanks again

This is what I was looking for:

"If you also want "doesn't exist" if B1:B2 is later (more recent) than
the first month/year represented in A1:A100, then:

=IF(OR(DATE(B2,B1,1)<A100-DAY(A100),DATE(B2,B1,0)A1-DAY(A1)),
"doesn't exist", MATCH(DATE(B2,B1,1),A1:A100,-1))


Cheers