Hi,
http://www.cisco.com/en/US/docs/ios/12_3t/fun/command/reference/cfrgt_02.html#wp1122007
says:
Some platforms have a hardware clock (calendar) in addition to a software clock. The hardware clock is battery operated, and runs continuously, even if the router is powered off or rebooted.
If the software clock and hardware clock are not synchronized, and the software clock is more accurate, use this command to update the hardware clock to the correct date and time.
To perform a one-time update of the hardware clock (calendar) from the software clock, use the clock update-calendar command in user EXEC or privileged EXEC mode.
So some routers should provide a hardware clock (calendar).
An example is XR 12000, see
http://www.cisco.com/en/US/docs/routers/xr12000/software/xr12k_r3.9/system_management/command/reference/XR12k_System_Management_Command_Reference_chapter4.html
But I have not found a complete list of such routers.
BR,
Milan
