You may experience that the BIOS defaults have been restored, the Time and Date has not been set message during power-up due to the CMOS battery depletion.
AC Power retains the computers standby power to reduce the power consumption of the CMOS batteries. If left without a power connection, the battery can deplete and lower the overall expected lifespan.
Expected Storage Environment and Shelf Life for Unused Batteries:
The battery has a shelf life of one year. To ensure good performance, it is recommended to use the battery within one year. If the storage time exceeds one year under standard storage conditions, it is possible that the CMOS battery depletes and not be able to retain the CMOS settings. Sometimes the CMOS battery may need to be replaced.