OK, so I just found that firmware 1.7 allows you to set time manually. To do so, first flash your controller with firmware 1.7. Then open a browser and type in the IP address (you can’t use app because the app doesn’t support firmware 1.7 which is a very old version). Go to Options, and check the ‘Change time?’ checkbox. Underneath that type in your current time, then password (opendoor), and click on Submit.
Once this is done, the RTC should remember the time, so when you upgrade to firmware 1.8.3 the time should be preserved.
If you can’t find the firmware 1.7, it’s in folder ‘osFirmwareUpdater/Firmware/OpenSprinkler_v1.4’.
Ideally you should have OpenSprinkler installed on a network with Internet access so it can perform NTP sync. Without NTP sync, the time will start to drift.