"Dave R." wrote...
Still not 100% sure what you want but it's getting late.

Ok - if your data (month & rate) is in A1:B20..

You can try this array formula

=IF(TEXT(INDEX(A1:A20,MAX(IF(B1:B20<"",ROW(B1:B2 0)))),"mmyy")
<TEXT(TODAY(),"mmyy"),"Update Exchange Rate","")

entered with CTRL-SHIFT-ENTER.
....

A bit long. The INDEX expression could be shortened to

LOOKUP(2,1/(B1:B20<""),A1:A20)

and not require array entry. However, if the OP just needs an indicator
whether there were an exchange rate in B1:B20 for every date in A1:A20, and
the dates in A1:A20 were date serial numbers, then the following array
formula would suffice.

=IF(OR(ISNUMBER(A1:A20)<ISNUMBER(B1:B20)),"Update Exchange Rates","")