[nSLUG] DST & tzdata

George N. White III gnwiii at gmail.com
Sun Mar 4 14:20:46 AST 2007

On 3/4/07, Mike <mspencer at tallships.ca> wrote:

> Slackware seems not to have a patch or equivalent for the new DST
> rules that GloboCop has ordered.  We're apparently being sucked
> into the propwash.
> So I fetched the tzdata files from:
>    ftp://elsie.nci.nih.gov/pub/tzcode2007c.tar.gz
> dated Mon Feb 26. Therein I note that the rules for America/Halifax
> haven't been updated to the new dates and there is no Zone... line for
> AST/ADT.  I think I understand from inference and the  zic(8) manpage
> how to fix things up but...

The files are tricky to read.  Look for the lines:

Rule    Canada  2007    max     -       Mar     Sun>=8  2:00    1:00    D
Rule    Canada  2007    max     -       Nov     Sun>=1  2:00    0       S

and note that America/Halifax is most recently (since 1974) set to
Canada.  After you have installed the files,

$ zdump -v America/Halifax | grep 2007
America/Halifax  Sun Mar 11 05:59:59 2007 UTC = Sun Mar 11 01:59:59
2007 AST isdst=0 gmtoff=-14400
America/Halifax  Sun Mar 11 06:00:00 2007 UTC = Sun Mar 11 03:00:00
2007 ADT isdst=1 gmtoff=-10800
America/Halifax  Sun Nov  4 04:59:59 2007 UTC = Sun Nov  4 01:59:59
2007 ADT isdst=1 gmtoff=-10800
America/Halifax  Sun Nov  4 05:00:00 2007 UTC = Sun Nov  4 01:00:00
2007 AST isdst=0 gmtoff=-14400

> Before I do that, does anybody know of any gotchas?  Is there an
> authoritative Canadian site that has updated tzdata files for Canadian
> zones?
> More technical yet: Is there any reason why I can't compile just the
> zoneinfo file for America/Halifax and put the binary into the
> canonical zoneinfo directory without doing the other zone files?  My
> own machine isn't a server and never moves out of the AST/ADT
> zone. Does anything important regularly use the zoneinfo files for
> *non-local* zones?  F'rgzample, if I get ntp running using a server in
> Boston or Ontario, will ntp reference my local zoneinfo files for for

Do you keep your hardware clock (the one that runs from battery while
the system is off) on UTC?  ntp should not touch the tzdata stuff --
normally it is only concerned with UTC, but if you keep the hardware
clock on localtime (e.g., in order to dual boot Windows), tzdata would
be used to translate to/from UTC when booting and shutting down to
read/set the hardware clock.

George N. White III <aa056 at chebucto.ns.ca>
Head of St. Margarets Bay, Nova Scotia


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