I've had issue with the dt620 of late...bad batch, though I've had good experience with them in the past. The issue was only if the timer was set to run dusk-to-dawn. It would turn off at midnight, and not turn back on until the following sunset. Quick fix was adding another start time of 12:01. It leaves a 1 minute gap at mid-night which isn't awesome, but good enough until you can get back with a replacement timer if you don't have one on the truck.
I've had good luck with the Tork timers I've used in the past. The rechargeable battery is a big plus. My main gripe is the smallish display and buttons. I'm still young, and my eyes are fine, but if the transformer is mounted at minimum height, it's hard to get down there and read that display. The 620's display is much better, as well as easier to program IMO. If I could make a request...would it kill you to put a backlight on the display? You know we do this stuff at night, right?