The Raspberry Pi does a great job keeping time as long as there is power.  After a power failure, the Raspberry Pi's time will not be correct.  Knowing the correct day/time is important for this project as the schedule determines when to turn on/off the system, so not having the correct time is a problem.

Two solutions come to mind:

1. Add a $10 RTC (real time clock) hardware module to the Raspberry Pi to keep time in the event of a power failure.
2. Use the first contact with a cell phone to set the time, from the cell phone's time.

The first option is easy but adds $10 to the system costs.

The second option works this way: After install, the Raspberry Pi comes up with some random day/time.  On first connection by a cell phone, push down the current day/time (from the cell phone) to the Raspberry PI.  As long as there is power, the Raspberry Pi will have the correct day/time.  The assumption here is the cell phone has the correct time, which is a reasonable assumption since most phones should have the correct time due to NTP.  In the event of a power failure (not likely since the system is off-grid and the battery/solar setup should keep the Raspberry Pi powered on 24/7), the Raspberry Pi's time will be messed up.  This means the watering schedule is messed up.  The system may water on the wrong day/times.  But, it will still water.  Once a cell phone reconnects to the system, the time will be corrected.