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up vote 7 down vote favorite

How can I use the date command to convert something like "monday week 40" into a ISO date?

I am playing with something like this:

date --date='monday week 40' +'%Y-%m-%d'

And the date I'm searching for would be 2011-10-03.

But my problem is that this date string is not valid, so I need another approach to solve this problem.

/Thanks

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The following link is about what determines on which day of the year a numbered week begins. Week numbering (Wikipedia).. It effectively explains why the 1st of Janunuary of this year is in ISO week 52 .. The %V format sequence used by 'user unknown' reports the ISO week number. – Peter.O Sep 1 '11 at 16:16

2 Answers

up vote 3 down vote accepted

Really ugly and probably works only with GNU date:

date -d "$( date -d "$( date +'%Y-01-01' ) +40 weeks") -$( date -d "$( date +'%Y-01-01' ) +40 weeks" +'%w' ) days+1 day" +'%Y-%m-%d'

Tested only for your 3 October example, may fail for some other cases.

Update: If you have a non eng locale you need to specify the output from the inner date to get to to work. (And %F just is YYYY-MM-DD).

date -d "$(date -d "$(date +'%Y-01-01') +40 weeks" +"%F") -$(date -d "$(date +'%Y-01-01') +40 weeks" +%w) days +1 day" +"%F"
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You missed to format the output from the inner date so the outer could use it correctly. – Johan Sep 1 '11 at 11:19
@Johan, I had no problem with using the default format. Maybe it is locale specific? I use en_US. Good point anyway. – manatwork Sep 1 '11 at 11:34
For the date I use the Swedish locale, so there is the difference. – Johan Sep 1 '11 at 13:09
Thanks for the locale adjustment. I have a custom date format and had the same problem.. I had almost sorted it out, but now you've posted it I'll use your tweak; it's better. – Peter.O Sep 1 '11 at 14:32
up vote 5 down vote

An alternative approach:

date --date "+$((40-$(date +%V)))weeks last monday"  +"%F"
  • 40 is the week you search for
  • date +%V returns the current week (35)
  • 40-35 = 5, which is the number of weeks to add
  • from there, seek the last monday
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good one, and it works with my custom date setting. – Peter.O Sep 1 '11 at 14:31

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