How to find difference in years between two dates?

[John Machin]

thebjorn wrote:
> John Machin wrote:
> > thebjorn wrote:
> > > John Machin wrote:
> > > > thebjorn wrote:
> [...]
> > > > Holy code bloat, Batman! Try this:
> > > >
> > > >     return now.year - born.year - (birthday > now)
> > >
> > > yuck :-)
> >
> > But this:
> >     return now.year - born.year - (birthday > now and 1 or 0) is not yuck???
>
> Correct.
>
> > > [...]
> > > > It's the irregular-size months that cause the problems. If you can work
> > > > out the months difference, then just floor_div by 12 to get the years
> > > > difference.
> > >
> > > I don't agree that the irregular sized months cause a problem in this
> > > case. They do cause a problem if you're asking "when is today + one
> > > month?", i.e. there isn't an unambiguous answer to that question in
> > > general (e.g. if today was January 31). We're asking a different kind
> > > of question though: "has it been at least one month since January 31?",
> > > the answer would be no on Feb 29 and yes on Mar 1.
> >
> > If a bank were paying you interest on a monthly basis, and you
> > deposited money on Jan 31 and pulled it out on the last day of
> > February, that would count as one month. This is what I call the "today
> > - yesterday == 1" rule. For computing things like duration of employee
> > service, you need the "today - yesterday == 2" rule -- on the
> > assumption that service counts from start of business yesterday to
> > close of business today. So hire date of 1 Feb to fire date of (last
> > day of Feb) would count as one month.
>
> You give a good argument that the concept of a month is fuzzy

Sorry,  I can't imagine where you got "fuzzy" from. Perhaps you mean
some other word. The concept is capable of being expressed precisely.

> I still
> don't believe that it makes the concept of a year fuzzy (or that the
> fuzziness of month needs to be accounted for when computing years).

The point is to ensure that (say) 24 months of employment and 2 years
of employment are determined in a consistent fashion.

>
> > Sorry, I don't understand. Why are you speechless? What would I want to
> > use the calendar module for? Apart from the leap() function and the
> > table of days in a month, the calendar module doesn't have any of the
> > functionality that one would expect in a general-purpose date class.
>
> Well, I thought replacing a 4 line function with 31 lines, 13 of which
> duplicated functionality in the standard library was overkill.

I think you missed the point that the lines I quoted were straight out
of a self-contained library that existed (in C as well as Python) way
before the datetime module was a gleam in Fred & the timbot's eyes.
Even if I had noticed a leap year function in the calendar module, I
would probably not have used it. The Python version of the module was
just a stopgap while I fiddled with getting a C extension going. The C
leap year function doesn't have any of that modulo stuff in it.

> I came
> up with this yesterday which seems sufficient?
>
>     def yeardiff(a, b):
>       y = a.year - b.year
>       if (a.month, a.day) < (b.month, b.day): # tuple comparison
>           y -= 1
>       return y

At least it doesn't blow up when b is leapyear-02-29. It just gives the
wrong answer when a is nonleapyear-02-28. E.g. it gives 0 years
difference  from 1992-02-29 to 1993-02-28 instead of 1.