For times before half past the hour the time is shown normally in black on a white background. For times after that and before the next hour the time is shown as a countdown to the next hour.

So the time is shown as: -
black on white - 15:01, 15:02, ..., 15:29, 15:30
white on black - 29:16, 28:16, ... 01:16, 16:00, 16:01