How to find the nearest symmetric time in the day

OK so I know my title is very misleading so I will explain to you all.

This is my homework:

A moment in a day is seen as symmetric if the hour number and the minute number is symmetrical (e.g 12:21)

The test required to input an hour number (00 -> 23)and a minute number (00 -> 59) combined to a string (e.g 23:59) and find the nearest symmetric moment, in our case, 00:00

Right now I’m far away and asking this question on my phone but my actual program is in my computer so I can’t put my approach right now. If you need it I will put it this evening with my problems on the way.

How can you do it? Thanks in advance

(P.e sorry if my English is bad I’m Vietnamese)