Why put a voltage divider between a battery and the battery pin of an RTC?
That module design with the voltage divider is TERRIBLE. 


It puts a permanent load on the battery when the 5V is off shortening the battery life significantly. 
In addition it reduces the usable voltage range of the battery again shortening its life span. 
And lastly the circuit forces a charge current into the battery when the 5V is on. Almost all coin cells are NEVER supposed to be charged. 


Here is how the circuit should be changed: