tztab file format for DST changes

[Christian Tremblay]

In the eastern time zone time changes for EDT and EST both in spring and fall occur at 02:00 AM.

Why is it that the hour field in tztab (the 2nd one)at which the change occurs is 3 in the spring and 1 in the fall ?

Should'nt these values be 2 for the change to occur at 02:00 AM ?

Thx
Chris

[A. Clay Stephenson]

No because the time change occurs at 01:01:59 EDT and transitions to 01:00:00 EST.

Use the attached Perl script, dst.pl to check your entries. It doesn't know anything about tztab directly but rather uses the same underlying libc functions that all the UNIX utilities like date use.

Invoke as dst.pl for full usage.

[James R. Ferguson]

Hi Chris:

No, the file is correct. The minute before the '3' hour mark is "1:59" at the transition point. We think of this at 2:00 AM.

For example, in April 2006 the time changed from 01:59:59 to 03:00:00. In October 2006 Eastern times will move from 01:59:59 to 01:00:00.

See the manpages for 'tztab(4)'.

Regards!

...JRF...

[Christian Tremblay]

Clay,I had already installed and run your script and it works fine, thank you.

But I was still wondering about those entries in tztab. I read the man page but it still wasn't clear.

So if I understand well that second field represents the "target" hour after the system changes time.

ie: In the spring it will change from 01:59:59 to 03:00 hence the 3 in the second field of tztab

likewise in the fall time will change from
01:59:59 to 01:00:00 hence the 1 in the second field.

Is this right or am I missing something still ?

thx
Chris

[James R. Ferguson · Accepted solution]

Hi (again) Chris):

You are correct.

Regards!

...JRF...

[Christian Tremblay]

Thanks Guys, have a good weekend...