If function with defined start and end date with performing calculation

[datbigdog]

Hello - ran a search and couldn't find a thread with my specific question.

I am looking to formulate an IF function that has a defined start and end date that also performs a calculation if the original if logical test is met.

For example:
I am projecting to save $100 over a 12 month period starting October 2018 and ending September 2019. There are specific monthly attributions for each month during that time period (say 8% during October, 10% during November etc. equaling the full 100% at the end of the 12 month period), however, I need the formula to work regardless of the start date as I will be capturing a large amount of data with differing start/end dates.

Is there a way to dynamically do this with a standard if function? Is there a better way?

Thanks in advance for any help you can provide.

 

[datbigdog]

As a follow-up, is there a way in the above example to create a date range (Oct 18 - Sept 19) and have the logical test reference any month contained within that date range to switch on the formula?