Add a month

[novin01]

Hi, this is probably a really simple question but...
How do you add a month to a datetime date in python? It would be nice
if you could do something like:

d = datetime.date(2006,2,17)
dm = datetime.timedelta(months=1)
d_new = d + dm

but timedelta doesn't have a 'months' setting. Am I missing some easy
method or do I have to code a work around myself??

Thanks in advance for the help!

 

[Raymond Hettinger]

[[email protected]]

Hi, this is probably a really simple question but...
How do you add a month to a datetime date in python? It would be nice
if you could do something like:

d = datetime.date(2006,2,17)
dm = datetime.timedelta(months=1)
d_new = d + dm

but timedelta doesn't have a 'months' setting. Am I missing some easy
method or do I have to code a work around myself??

You need to code you own version.

It was not included with datetime.timedelta() because the definition is
ambiguous (i.e. what date is one month after Jan 30th?).


Raymond

 

[Fredrik Lundh]

Hi, this is probably a really simple question but...
How do you add a month to a datetime date in python? It would be nice
if you could do something like:

d = datetime.date(2006,2,17)
dm = datetime.timedelta(months=1)
d_new = d + dm

but timedelta doesn't have a 'months' setting. Am I missing some easy
method or do I have to code a work around myself??

what do you expect d_new to be after the operation ? if the answer
is date(2006,3,17), what's date(2006,1,31) plus one month?

</F>

 

[Daniel Dittmar]

what do you expect d_new to be after the operation ? if the answer
is date(2006,3,17), what's date(2006,1,31) plus one month?

Traceback (most recent call last):
File "<stdin>", line 1, in ?
ValueError: date domain error

in analogy to math.sqrt (-2)

Daniel

 

[Paul Boddie]

what do you expect d_new to be after the operation ? if the answer
is date(2006,3,17), what's date(2006,1,31) plus one month?

February 31st, of course:

http://sql-info.de/mysql/gotchas.html#1_14

;-)

Paul

 

[=?ISO-8859-1?Q?Gregory_Pi=F1ero?=]

Here's how I do it:

def monthify(anint):
if anint%12==0:return 12
else:return anint%12

import datetime
d=datetime.datetime.today()
dplus1month=datetime.datetime(d.year,monthify(d.month+1),d.day)

We need monthify because adding 1 to 12 is bad otherwise!

-Greg

 

[=?ISO-8859-1?Q?Gregory_Pi=F1ero?=]

Actually, no wait, that's bad. It doesn't increment the year.

Does anyone have a simple way to code this?

-Greg

 

[Carsten Haese]

Actually, no wait, that's bad. It doesn't increment the year.

Does anyone have a simple way to code this?

-Greg

I don't know if this qualifies as simple, but it seems to work:

def addmonths(thedate,months):
"Add <months> months to <thedate>."
import datetime
y,m,d = thedate.timetuple()[:3]
y2, m2 = divmod(m+months-1, 12)
return datetime.date(y+y2,m2+1,d)

Note that the date constructor will raise an exception if the result
happens to be an invalid date such as February 30th or November 31st.

-Carsten

 

[Jarek Zgoda]

Paul Boddie napisa³(a):

February 31st, of course:

http://sql-info.de/mysql/gotchas.html#1_14

MS SQL Server documentation marks dateadd() result as non-deterministic
in Transact-SQL reference. Adding 1 month to jan 31st gives feb 28 or
29, depending on year. It's not that bad, providing you know this
behaviour.

 

[=?ISO-8859-1?Q?Gregory_Pi=F1ero?=]

Ok, 30 minutes later, here's my best solution.

http://www.answermysearches.com/index.php/how-to-add-months-to-a-date-in-python/53/

(Posted on my website to make sure tabs are kept.)

Would someone mind double-checking my logic before I put this into
production next Tuesday?

-Greg