Birthday Expression (No Future Dates Allowed)

I have a Birthday column in a USER PROFILE table. The field type is set to DATE. How do I set it up so that it does not allow for future dates to be selected. Any best practices on how to set this up?

Set Valid If expression to:

([_THIS] <= TODAY())

Thanks Steve. Expression makes sense.

Although, I’m hoping to learn more about [_THIS] and [_THISROW] as I feel like I’m not as strong with those particular expressions yet.