A Cisco IOS firewall keeps an internal system clock that can be used for Syslog time stamps, certificate time stamps, and so on.
When an IOS firewall router is initialized, the system time is set from a hardware time clock (system calendar) in the router. The clock is powered by a battery in the absence of regular power.
The system clock is always based on Coordinated Universal Time (UTC). UTC was previously known as Greenwich Mean Time (GMT). You can set the system time using two different approaches:
Manually— You set the time and date on the firewall along with the time zone and specify whether to observe daylight savings time. With manual configuration, the firewall clock is only as accurate as ...