If it is valid, pls revert back "valid",
If it is not valid, pls "invalid"
Not sure do i have to use the " datetime" tool for this function or not.
Thanks so much.
Instead of datetime functions, try Regex_Match function in Formula tool like:
IF REGEX_Match([Field1],"^\d{4}-\d{2}-\d{2}$")
THEN "valid"
ELSE "invalid"
ENDIF