Even upside down, a bat can continue to use its digital watch, because some hours are the same even when viewed in reverse. For example, this is what happens with 12:21. Let's call these hours reversible.
What is the shortest interval between two reversible hours?
The only times that can possibly have the colon in the right place even upside down must have 2 digit hours. Therefore 10:01, 11:11, 12:21. Each spaced 70 minutes apart.
On a 24 hour clock 15:51, 20:02, 21:12, 22:22 give a few more spacings of 70 minutes but none shorter.
Colon included
