In the Fall, how is the time updated for DST?

[DISA SA]

I've got the latest tztab patch and everything. I'm curious when we "fall back" an hour in the fall for DST, how does the system handle it? I'm assuming it just slows the clock down for a period of time until it's an hour behind, or does it just jump from 0159 straight to 0100? If it does just slow down for a bit, how long does it take? I'd just like to learn a bit more about the actual process. Any information would be greatly appreciated.

thanks!

[James R. Ferguson]

Hi:

UNIX recons time in epoch seconds --- the number of seconds since January 1, 1970 expressed in Universal Time Coordinates (UTC) otherwise known as GMT.

Localtime zones are +- UTC with or without a Daylight Saving factor.

The first thing to remember is that the incrementing of the elapsed seconds since the Epoch continues unabated with the occassional imposition of a leap second or the stalling of the count for one leap second.

The 'tztab' file defines the local rules for offsetting what you perceive as the correct time, along with the date and time that this offset will be added or subtracted from the epoch seconds.

To understand the rules as expressed in the 'tztab' see the manpages for 'tztab(4)'.

To understand how the 'cron' daemon deals with the preceived time change, see the manpages for 'cron(1M)'.

The actual counting clock neither pauses nor jumps except for the aforementioned (rare) leap second.

Regards!

...JRF...

[Dennis Handly]

>JRF: with the occasional imposition of a leap second or the stalling of the count for one leap second.

There is no such thing as automatically added leap seconds for HP-UX. Any corrections would be due to NTP.

[James R. Ferguson]

HI (again):

>Dennis: There is no such thing as automatically added leap seconds for HP-UX. Any corrections would be due to NTP.

Yes, that's true. I can see, however, how some readers might not understand that it is NTP that handles this. After all, every sane person runs 'xntp'. I should learn to be more careful with my writing.

...JRF...