I haven't used Facer for a while, but I know that I had found one solution with a formula that add a leading zero when under 10 and it works with both 12h and 24h systems.
(but I forgot where I found it, so I can't properly acknowledge the person that found it, but I didn't create it).
It's set to show 00 as midnight, I don't know what you should change to use 24 as midnight.
Formulas using booelean logic are too complex for me, especially when the documentation think we already know what || and && means
By the way, I still think that we should have a #DB# tag with leading zero and the Facer documentations are still seriously lacking content for people who aren't used to use formulas and stuff...