i've written it out now to make it clearer, if the first day of 1726 was Mar 25th, the last day was Mar 24th, so the first day of 1727 was March 25th, the first february in 1727 was therefore 15 months after Nov 1726, is that not right?
Sure this is not where I first encountered it, but http://spreadsheetpage.com/index.php/file/C37 seems to have the XDATE (extended date) macros which allow you to measure between dates and other stuff.
Try the links Dot, they are designed to correct Excel spreadsheet date flaws. You install the macros and then can use them when writing the formula in the cell. Easy enough to use but you need to ensure the Excel macro security is at a level to allow you to run them.