4 ms·

Re: Linux Date Command gives wrong year with +%g

31 points by billpg 17y ago

houseabsolute 17y ago
Probably "wrong" here should be in scare quotes. It's not the wrong year; it does return the correct ISO year. It's just that ISO year is not what you want so you should be using %y rather than %g.
- telemachos 17y ago
  Check the actual link: that's exactly what the email linked to says. (And the title here is simply the title of the email.) I'm assuming that (1) the poster is warning others about this or (2) he finds it amusing that the response to "The year is wrong" is (roughly) "No, that's how the standard defines year. The output is right relative to that standard. Dates and times suck." (the last sentence is implied) or (3) both.
- access_denied 17y ago
  And thusly the "problem" is not one of Linux but a funny characteristic of our universe.
pierrefar 17y ago
It's wrong in the sense that not the obvious result users expect. But it's 100% technically right. It's always tough to deal with this kind of situation. Do you document it all over the place and hope users read it? Do you parse the command when it's first created and warn the user that %g is different than %Y? Do you just ignore the issue and deal with the small number of emails at the beginning of every year? I think a combination of docs + automated checks is best depending on the circumstances.
zackattack 17y ago
Can someone please explain what ISO years are and when the correct time to use them is?
- indiejade 17y ago
  International Standards Organization years; standardizing date-time formats across various UNIX systems. Many people from Europe, for example, prefer the month to be stated after the day of the month. "2 January YYYY" whereas in other places (in formal writings) people prefer the month to be written first: "January 2, YYYY". Sometimes (if, for example, running virtual host) they can appear to be a mix-up since there's quite a bit of ambiguity within the %m%d%H%_M% or "%m%d%H(M)" string sequences. For sources, see also "errata" on intuitive.com/wicked when the correct time to use them is? Basically amounts to personal preference since options can be formatted.
  - calcnerd256 17y ago
    That doesn't really answer the question they were trying to ask, but other replies do.
- blasdel 17y ago
  They match up with ISO week numbers, where week 1 always starts on a Monday. I regularly see ISO weeks used to mark manufacturing dates, where they often offer the right levels of precision, as many processes run on week-long cycles. Often in stampings the year will be replaced by a letter code with an epoch particular to the company.
- CUViper 17y ago
  http://en.wikipedia.org/wiki/ISO_week_date http://en.wikipedia.org/wiki/ISO_week_date The ISO week date system is a leap week calendar system that is part of the ISO 8601 date and time standard. The system is used (mainly) in government and business for fiscal years, as well as in timekeeping.
edge17 17y ago
when would you actually want to use an ISO year though? just to claim ISO compliance?
- skoob 17y ago
  Things get weird if you use weeks as a unit and then also display the year. For example, if you have an event that took place on 2010-01-01, then iso_week_of(2010-01-01) will return 53 and year_of(2010-01-01) will return 2010. So your wonderful app might display "Week 53, 2010", which makes it look like it's the end of 2010.
bcl 17y ago
Here is how strftime on OSX defines it: %G is replaced by a year as a decimal number with century. This year is the one that contains the greater part of the week (Monday as the first day of the week). %g is replaced by the same year as in ``%G'', but as a decimal number without century (00-99). Makes sense to me. People should read the man pages more often before they jump to conclusions. Granted, that is more clear than man date on Linux (on OSX it doesn't define the format variables): %g last two digits of year of ISO week number (see %G) %G year of ISO week number (see %V); normally useful only with %V %V ISO week number, with Monday as first day of week (01..53)
calcnerd256 17y ago
%g tells you the year (modulo the century) in which the current week's Monday took place

31 points by billpg 17y ago

Probably "wrong" here should be in scare quotes. It's not the wrong year; it does return the correct ISO year. It's just that ISO year is not what you want so you should be using %y rather than %g.

telemachos 17y ago
Check the actual link: that's exactly what the email linked to says. (And the title here is simply the title of the email.) I'm assuming that (1) the poster is warning others about this or (2) he finds it amusing that the response to "The year is wrong" is (roughly) "No, that's how the standard defines year. The output is right relative to that standard. Dates and times suck." (the last sentence is implied) or (3) both.
access_denied 17y ago
And thusly the "problem" is not one of Linux but a funny characteristic of our universe.

pierrefar 17y ago

It's wrong in the sense that not the obvious result users expect. But it's 100% technically right. It's always tough to deal with this kind of situation. Do you document it all over the place and hope users read it? Do you parse the command when it's first created and warn the user that %g is different than %Y? Do you just ignore the issue and deal with the small number of emails at the beginning of every year? I think a combination of docs + automated checks is best depending on the circumstances.

zackattack 17y ago

Can someone please explain what ISO years are and when the correct time to use them is?

indiejade 17y ago
International Standards Organization years; standardizing date-time formats across various UNIX systems. Many people from Europe, for example, prefer the month to be stated after the day of the month. "2 January YYYY" whereas in other places (in formal writings) people prefer the month to be written first: "January 2, YYYY". Sometimes (if, for example, running virtual host) they can appear to be a mix-up since there's quite a bit of ambiguity within the %m%d%H%_M% or "%m%d%H(M)" string sequences. For sources, see also "errata" on intuitive.com/wicked when the correct time to use them is? Basically amounts to personal preference since options can be formatted.
- calcnerd256 17y ago
  That doesn't really answer the question they were trying to ask, but other replies do.
blasdel 17y ago
They match up with ISO week numbers, where week 1 always starts on a Monday. I regularly see ISO weeks used to mark manufacturing dates, where they often offer the right levels of precision, as many processes run on week-long cycles. Often in stampings the year will be replaced by a letter code with an epoch particular to the company.
CUViper 17y ago
http://en.wikipedia.org/wiki/ISO_week_date http://en.wikipedia.org/wiki/ISO_week_date The ISO week date system is a leap week calendar system that is part of the ISO 8601 date and time standard. The system is used (mainly) in government and business for fiscal years, as well as in timekeeping.

edge17 17y ago

when would you actually want to use an ISO year though? just to claim ISO compliance?

skoob 17y ago
Things get weird if you use weeks as a unit and then also display the year. For example, if you have an event that took place on 2010-01-01, then iso_week_of(2010-01-01) will return 53 and year_of(2010-01-01) will return 2010. So your wonderful app might display "Week 53, 2010", which makes it look like it's the end of 2010.

bcl 17y ago

Here is how strftime on OSX defines it: %G is replaced by a year as a decimal number with century. This year is the one that contains the greater part of the week (Monday as the first day of the week). %g is replaced by the same year as in ``%G'', but as a decimal number without century (00-99). Makes sense to me. People should read the man pages more often before they jump to conclusions. Granted, that is more clear than man date on Linux (on OSX it doesn't define the format variables): %g last two digits of year of ISO week number (see %G) %G year of ISO week number (see %V); normally useful only with %V %V ISO week number, with Monday as first day of week (01..53)

calcnerd256 17y ago

%g tells you the year (modulo the century) in which the current week's Monday took place