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?

Source:Rudolpho Korchan.

(In reply to Colon included by Jer)

An implication is made that a "reversible hour" is a valid time as displayed in seven-segment display format that turned 180-degrees still represents the same valid time.

In some digital timepieces that display military time, 24:00 is represented as 00:00. Given the definition of "reversible hour", there are only a few times that belong to the set:
00:00, 01:10, 02:20, 05:50, 10:01, 11:11, 12:21, 15:51, 20:02, 21:12 and 22:22. For a military-time reversible hour, the shortest interval is -- as Jer pointed out -- 70 minutes.

For non-military time, the set is much smaller:
01:10, 02:20, 05:50, 10:01, 11:11, 12:21. As with military-time, several of the short intervals are 70 minutes apart, yet 12:21 and 01:10 are only 49 minutes apart.

Posted by Dej Mar on 2015-12-09 07:58:20