Thanks for all your replies!
Currently, I have the Photon connected to both a 5V supply (it’s switch reg brings the Vcc to 3V3) as well as a CR2032 Li-ion battery on the Vbat pin. The goal is so that I can immediately inquire the current time whether I can connect to Spark cloud or not.
In my application, I need to be able to know the current time stamp as a device start-up condition. This means that even if cloud is available, I cannot afford to wait until the cloud is connected (best case I had was about 3 seconds from power-up and worse case it could be 10’s of seconds to connect to the Particle cloud).
With the Vbat connected to a battery, I still could not figure out how to use the RTC as a real RTC (like the DS1307). the time library does not allow me to set the RTC (using setTime()). Even with the battery supply on, once I power-cycle the Photon, the default time immediately went back to sometime in 1913.
Is there a way to use the RTC in Photon’s P0 as a real RTC?