The input of 1/5/2015 in the first table was actually an error which I've changed to 5/5/2015. Essentially what I want to do is for each of the start dates, calculate the cumulative sum of any amounts that haven't expired. So for the first four dates, it is just a running cumulative sum because none of the amounts have expired. But at 5/5/2015, two of the previous amounts have expired hence a cumulative sum of 30. Same for the last date, where the same two have previously expired and you have the additional amount as of 1/6/2016 therefore 40.
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