On Mon, Nov 02, 2009 at 10:52:40AM +0000, Jasen Betts wrote: > what's the absolute value of '1month -30 days'::interval > > if I add it to the first of march it goes forwards if I add it to the In march a month has 31 days. > first of february if goes backwards. if I add it to the first of april 29 or 28 days. > it goes nowhere. 30 days so the math below is 0. > select '2008-02-01'::date + '1month -30days'::interval; > select '2008-03-01'::date + '1month -30days'::interval; > select '2008-04-01'::date + '1month -30days'::interval; > > I'm guessing that '1 month 30 days' is the only mathematically > consistent answer, not that it's likely to be useful. A month is not fixed width. It depends on which month you're talking about. And then there is http://en.wikipedia.org/wiki/Month. Ow. Having given it some more 1am thinking the above probably works out to: select '2008-02-01'::date + '1month -30days'::interval; 2008-02-01 + 1 month = 2008-03-01 2008-03-01 - 30 days = 2008-01-31 (feb has 29 days) select '2008-03-01'::date + '1month -30days'::interval; 2008-03-01 + 1 month = 2008-04-01 2008-04-01 - 30 days = 2008-03-02 (march has 31 days) select '2008-04-01'::date + '1month -30days'::interval; 2008-04-01 + 1 month = 2008-05-01 2008-05-01 - 30 days = 2008-04-01 (april has 30 days) I'm guessing (no postgres avail to me atm) and half asleep so my math my be boogered. In the end, though, it all makes sense. :) -- "A search of his car uncovered pornography, a homemade sex aid, women's stockings and a Jack Russell terrier." - http://www.news.com.au/story/0%2C27574%2C24675808-421%2C00.html -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
