Re: How to find difference in years between two dates?
- From: "John Machin" <sjmachin@xxxxxxxxxxx>
- Date: 27 Jul 2006 17:27:24 -0700
thebjorn wrote:
John Machin wrote:
thebjorn wrote:[...]
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.
and the second to last date in January plus a month is..?
The last day in February, according to me and also according to the
website that you mention later, e.g.
"""
Added 1 monthFrom date: Thursday, January 30, 1992
Resulting date: Saturday, February 29, 1992
"""
The precise rule is that if the original day is later than the last day
of the target month, clip it to that last day.
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'm sure it doesn't. You might want to look at the assembly your C
compiler produces for a modulo-power-of-2 operation...
I don't need to. AFAIR just about every compiler I've used (BDS C on a
Z80 is a likely exception) has generated "& mask" (with extra mucking
about if the first operand is signed). Leap year calc involves % 100
and % 400. Factoring out the powers of two leaves 25. Now tell me how
I'm managing that without a modulo operation.
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.
I believe you're mistaken (but feel free to correct me), my grandmother
is born on Feb 29 and a quick call to my dad verified that they
celebrated it the day after the 28th (unless Mar 1 was a Monday ;-).
http://timeanddate.com/date/duration.html also seem to agree with my
current understanding, just as a datapoint if nothing else.
hmm ... let's look at more than 1 data point from that website:
easy cases Jan 1993 to Feb 1993:
Jan 10 to Feb 9: 30d (excluding last day) 1m 0d (including last day)
Jan 10 to Feb 10: 1m 0d (excluding last day) 1m 1d (including last
day)
Jan 10 to Feb 11: 1m 1d (excluding last day) 1m 2d (including last
day)
looks reasonable, consistent, no anomalies; note how 1m 0d (excluding)
is "predicted" from the right and below.
Now try from Jan 31 1993:
Jan 31 to Feb 27: 27d (ex) 28d (in)
Jan 31 to Feb 28: 28d (ex) 1m 1d (in)
Jan 31 to Mar 01: 1m 1d (ex) 1m 2d (in)
So 1 day short of 1m 1d is not 1m 0 d??? I'd call this unreasonable,
inconsistent, anomalous -- especially when on the same website you do
1993-01-31 plus 1 month, it gives you 1993-02-28 (as I'd expect).
.
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