Water bill breakout spreadsheet?

[Mathew Deines]

Do any of you happen to have a spreadsheet that will calculate a particular tenants portion of the water bill considering the number of days they have occupied the unit? For example: the tenant moves in 9/24/2015 but the bill for the building goes from 8/7/2015 to 10/06/2015. 

I have a sheet that I am working with but I cannot seem to get the correct number when trying to add in the number of days the tenant occupied the unit. Any assistance would be greatly appreciated. Thanks!