Just a bit of feedback, the formatting doesn't all work correctly. %mm does, but %m doesn't (if you don't want to fill a leading zero). Same with %ss working but %s does not, and %hh vs %h.
It would be VERY handy if there was some way to indicate the rollover from countdown to count-up. I'd like to use this for my AFK scene, but it'd be preferred if the counter gained a - or (better yet) changed the 'Countdown Final Text' box to be a second Format box.
This would allow changing the text from (for example) "Ferret will be back in: XX:XX" to "Ferret is overdue to return by: XX:XX". It would also allow eliminating the 'Countup When Done' checkbox entirely... if you don't want a countup, just don't include any string-replacements in the text you enter in the 'Count Complete Text' box.