This brain puzzle was harder in practice than it looked. My brain feels a bit fitter than it was a couple of days ago. Well done!

Donuel, is it 5 or 6 without 24 hour time?

01:10, 02:50, 05:20, 10:01, 11:11, 12:51

(assuming that 12:00 is not shown as 00:00)

I like that if you put 1251 and 1521 together you get a palindrome - 12:5115:21.