One posibility is
search: <\!-- \RITEMDATE: =....-..-.. ..:..:..= -->
replace: \2
Variations could ignore the open and close comments to find the tag in the text as well, ignore the case of the tag, be more relaxed about the type and amount of whitespace, and be more restrictive about the allowed digits within the date.
Thanks, that should work fine. Related to this, I couldn't seem to find the documentation about how the replace syntax works (without parentheses as in regex). Does the 2 relate to the 2nd "repetition operator"?