DS3231 on Battery Power and Use of Aging Register
The battery supply is intended principally to keep the timekeeping going while the main supply is absent. It is possible to use I2C while the clock is powered by VBATT, but the load on the battery increases considerably. (It is unclear from the data sheet if the local I2C pullup resistors are powered in this mode from the battery or from another source.) In any case, if there is power to run I2C then it is clearly better to use this to power the DS3231.

The Aging Offset register allows the timebase to be adjusted as an offset from the factory calibrated value. Unless you have a VERY accurate time reference, I suggest that you should leave this alone.