No, the time and date registers are only reset upon first reading of the I2C registers after a power source has been applied. If you have two power sources Vcc and Vbatt connected, the time and date registers will not be reset as long as at least one power source satisfies specifications. The time and date registers are only reset when reading them after the internal oscillator has been restarted. The internal oscillator is shut down when all power sources are removed and restarts when a valid power source is applied.