Resolve Cisco's %ENVMON-4-RTC

%ENVMON-4-RTC: The Real Time Clock may have battery problem or it has not been set to the current time

You may receive the above error message if a battery in a Cisco 1841 or similar router has been replaced or gone flat.
Despite what the Cisco website says the battery is user replaceable.
If you have replaced the battery you can easily clear the error by setting the Real Time Clock again which is powered by the battery.
Steps to resolve:
1. Update the clock either manually setting the time or use NTP
Router#clock set 23:59:00 20 Jan 2000
2. Update current clock to internal Real Time Clock.  
Router#clock update-calendar

Extra Clock Options:
Router#clock ?
read-calendar Read the hardware calendar into the clock
set Set the time and date
update-calendar Update the hardware calendar from the clock