I know the algorithm used by Excel to calculate the YEARFRAC(startDate, endDate, basis) for basis=1. Excel calls the method "act/act". A Java re-implentation of Excel's algorithm can be found at finmath.net together with a spreadsheet comparing several day count conventions.
It appears as if Excel does not implement "ACT/ACT AFB" and definitely the method is not "ACT/ACT ISDA" (which is the most common one I assume).
I know all the algorithms (see also Implementation of the YEARFRAC). I just have the simple question:
What is the name of Excel's algorithm?
Or is it just "Microsoft's proprietary ACT/ACT".