catch command

What am I doing wrong?

I’m under the impression that err is valued only if something blows up.

catch {something blew up} err

What I’m trying to do is look for a bad or missing date so I thought that:

set txa4 {}

set err {}

catch {set data [clock scan $txa4]} err

err would be valued only if the date was missing or invalid and the scan command would blow up. But when I check $err, it seems to be valued with something.