The current OpenSprinkler design uses PCF8563. The firmware can auto-detect the chip:;
https://github.com/OpenSprinkler/OpenSprinkler-Firmware/blob/master/I2CRTC.cpp#L46
so you can use any of PCF8563, DS1307, and MCP7940.
You are right that DS1307 requires at least 4.5V VCC. So the previous version of the OpenSprinkler that used DS1307 has its VCC tied to +5V, whereas PCF8563 can work with 3.3V so its VCC is tied to 3.3V.