I have a %_date% in my filename when exporting my library. But it outputs in mmddyyyy format. I'd prefer YYYY.MM.DD format. Is it possible to do that?

Thanks

Yes, there are some solutions, here is one from DMY to YMD.
From "24.04.2008" to "2008.04.24" with
$regexp(%_date%,(\d\d)\.(\d\d)\.(\d\d\d\d),$3.$2.$1)

Here is another one from MDY to YMD.
From "04.24.2008" to "2008.04.24" with
$regexp(%_date%,(\d\d)\.(\d\d)\.(\d\d\d\d),$3.$1.$2)

Better set the Date Time configuration of your computer system accordingly to ISO 8601 in order to use always the international standard format for dates YYYY-MM-DD.

DD.20080424.1835.CEST
Edit.DD.20110224.0839.CET
Edit.DD.20110420.1042.CET