Formula to calculate after a set amount of days

[whatdoido]

Afternoon!

I've been tearing my hair out over this, and I hoping for some enlightenment from everyone here!

I would like a formula that will calculate the number of days after a certain date.

Basically we offer a storage service, which has an incentive of a number of days (different for each customer) free storage per item, the number of days free is presented in a cell for the formula to reference (so it can be changed easily later down the line). We bill monthly, so I was hoping for a formula that will take into consideration the free period, but will not keep deducting those seven days each billing period?

I hope I have explained that enough to make some sense

I have attached a screen shot below to help.

Column 'K' is where the formula will be placed (the formula there doesn't work, so those values are incorrect)
Cell 'I4' is the free period
Column 'I' is the date of the last bill sent to client or if after that date the date the item arrived in depot

I added column 'R' as a work around, which I don't know if it will help or not, it is a basic formula "=IF(B9<>"",B9+$I$4,"")"


Any advice or criticism is welcome!!
Thank you

 

[Fluff]

Is this for Excel or Google Sheets?

 

[awoohaw]

Thank you!

The formula in E5 seems to work, however if the date difference is less than the free days it produces a minus value.
Could an if function be added to produce a '0' if number of days to bill is less than the free period?

Many thanks

why would it throw a zero? Isn't the date the billing date prepared exactly 1 month after the begin date?