Some platforms have a hardware clock that is separate from the software clock. In Cisco IOS software syntax, the hardware clock is called the “calendar.” The hardware clock is a battery-powered chip that runs continuously, even if the router is powered off or rebooted. After you set the hardware clock, the software clock will be automatically set from the hardware clock when the system is restarted or when the clock read-calendar EXEC command is issued. The time specified in this command is relative to the configured time zone.
Commands and descriptions:
clock read-calendar: Performs a one-time update of the software clock from the hardware clock (calendar)
clock set: Sets the software clock.
clock summer-time: Configures the system time to automatically switch to summer time (daylight saving time)
clock timezone: Sets the time zone for display purposes.
clock update-calendar: Performs a one-time update of the hardware clock from the software clock
clock calendar-valid: To configure a system as an authoritative time source for a network based on its hardware clock (calendar), use the clock calendar-valid command in global configuration mode.
ntp update-calendar: To periodically update the hardware clock (calendar) from a Network Time Protocol (NTP) time source, use the ntp update-calendar command in global configuration mode.