date question

how do i go 6 months back?

so today is Feb 17, 2010 I use datevec to transform to Feb 1,2011, I can just do datenum-6*30 and transform that to Aug 1,2010 but is there a better way to do this? I want to make the intra month assumption above and get the first of the month 6 months back from the first of whatever month I am in.

d1=datevec(date())
d1(3)=1
d2=datenum(d1)-6*30
d2=datevec(d2)
d2(3)=1
datestr(d2)

thanks for you help

Answer 1

[Walter Roberson]

d1 = datevec(date());
datestr([d1(1)-1 d1(2)+6 1 0 0 0])

This uses the trick that you cannot go negative on months, but you can go back a year and forward 6 months from there.

Do not use -6*30 as that does not take in to account the length of months and will not handle leap-years properly.

ps: today is Feb 17, 2011 not 2010 :)

Answer 2

[Leah]

that's exactly why i didn't want to do it myway. The problem is you know in the example you are crossing a year, so that doesn't work. I want to be able to do this for every day.

bah! 2011, like how i fixed the second one ;)

[Walter Roberson]

I tested my code before posting; it works fine whether years are crossed or not.