I'm working in Excel 2007.

I have two columns of dates, the first being a "buy" date and the second
being a "sell" date for the item in that row. If the data is valid, obviously
each date in the second column should be later than the corresponding date in
the first column.

I'd like to create a function that will compare each date in the first
column to the adjacent date in the second column, and tell me if the data is
valid or if there is a date-pair in which the "buy" date falls after the
"sell" date. I don't need this comparison returned for each pair, I just need
to check the columns as a whole and get a "VALID" OR "INVALID" value.

I'm thinking that this needs to be some sort of composite function, maybe
involving a COUNTIF(), but I just can't figure out how to work it.

Thanks for the help!