I've done a lot of additional research regarding this and the thing I've found is that all sources regarding this subject avoid this kind of calculation. Even the ISO standards avoid this.
There is no real clear definition of how one should calculate or deal with this kind of date differences. If differences between dates are calculated it is never done in "years" or "months".
Nevertheless, in the
updated Holiday Counter Plugin I've used method B as described in this thread (and seemingly prefered by most, not always for the good reasons but still...). Method A is available as well, as also two analytical methods where months and years have a fixed amount of days.
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