Good job
BTW, Arduino Nano is using low precision crystal (>100ppm) for clock (and every other timing), 10pm would be more appropriate (for AP... but it wouldn't hurt for visual as well).
Of course, this is way better than free running DC motor you had before.
Deviation period of 15 minutes you are mentioning suggests PE (in gearbox probably.. what is the rotational rate of motor/gearbox shaft?).