When NTP is configured on a device, there is what is called a poll interval. This interval is dynamic and as client and server become better synced, and there aren’t any dropped packets, this interval increases to a maximum of 1024 seconds. If you change the time using the
clock set command, the time you set will become the new time. However, when the poll interval is exhausted, the device will re-sync with the NTP server. So any changes you make manually will be over-ridden at the next poll interval.
To change the time zone of a router, use the
clock timezone _zone hours-offset _ command where
* zone is the name of the zone to be displayed - this is just a label that you can define
* hours-offset is how many hours difference from UTC
You can find more information about this command here.
NTP always communicates time in UTC. If you have a time zone configured on your Cisco device and it is configured as an NTP client, then it will receive the time in UTC and will convert the clock to the local time zone. However, SYSLOG messages will always indicate UTC time by default, even if you have configured a different time zone. In order to have SYSLOG messages display the local time instead of UTC time, you can achieve this with the following command:
service timestamps log datetime localtime
The hardware clock of a Cisco device is a hardware chip on the motherboard of the device with a rechargeable backup battery. The hardware clock functions separately from the software clock and its main purpose is to retain the time and date information after a reboot or an extended period of time where the device is powered down.
The software clock is the clock that functions during normal operation. It is maintained in memory using the CPU as a “timekeeper”. It is this clock that can be updated and synchronised with an NTP server on the network.
These two clocks work together to maintain the most accurate time. The software clock can be updated periodically from NTP, and it in turn updates the hardware clock at regular intervals. When the device is rebooted, the software clock is synchronised with the hardware clock to obtain its initial time when it begins to function.
The two clocks can be managed separately however. More information about related commands can be found here.
I hope this has been helpful!