Intervals to calculate compounding interest

Hi,
The attached chart uses intervals to calculate compounding interest - but because the number of days in a month varies, using whole numbers for the interval becomes less and less accurate as the timeframe increases. Ideally I would like to make this work by using numbers to 2 decimal points i.e 30.42 instead of just 30.
PLEASE NOTE that I know there are formulae that will do this without an extensive spreadsheet but this is NOT what I want. I DO WANT to show the extensive spreadsheet, just with a more accurate way to automatically select every 30th (or more accurately 30.41667th) date / number and add the accumulated interest to the principal etc. Many thanks.