1) What firmware are you using? The OpenSprinkler firmware, or your own program?
2) Is your super capacitor charged to at least 2V, so that it can keep the RTC chip going when power is off?
I just verified that with the current OpenSprinkler firmware, power on, NTP sync, then power off, keep the unit away for 1 hour and power it on again, the LCD shows the correct time right away before NTP sync. So it’s working as expected.