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Operating System - Tru64 Unix
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DST and NTP Interaction

Joseph P. Smith
Regular Advisor

DST and NTP Interaction

Good Morning,

DS20, ES40 and ES45 running
Tru64 5.1B-3 (PK4)

DST patch is loaded on DS20 and ES40, and will be loaded on ES45 real soon now.

At 00:30 every day the three nodes synch to an MS 200x time server via cron job
30 00 * * * /usr/bin/ntp -s -f timeserver 0

Our IT group plans to test during daytime on the DS20 as follows:
- use btcreate to backup all file systems
- move date/time ahead to 11-March-2007 1:55:00
- observe time shift
- test applications

After success (?) at that date-time,
- move date/time to 04-November-2007 01:55:00,
- observe time shift
- test applications
- boot btcreate tape and restore all file systems

We won't be running tests at 12:30 A.M.

As we only synch at midnight my guess is that there's no interaction between NTP and shifting time manually. Correct?

Thanks for your help. ,,, Joe S.
Martin Moore

Re: DST and NTP Interaction

I'm not entirely sure about the NTP issue (perhaps someone else will chime in). If you are running ntpd or xntpd, it would seem prudent to stop the daemon during your tests. My primary reason for replying is to address one step in your procedure:

- move date/time to 04-November-2007 01:55:00

This won't work as you expect. The time 01:55:00 (or any other time in the interval 01:00:00 through 01:59:59) occurs *twice* on the day that DST ends. This is because at 02:00 Daylight time, the time reverts to 01:00 Standard time. If you use the date command to set the date to 01:55 on Nov. 4, it will unfortunately choose the second occurrence of 01:55 (i.e., the one *after* the time change) and your test will not be successful.

There are two ways around this:

1. Set the time to 00:55, which is not ambiguous, and wait longer.

2. Go through a timezone such as GMT to force the time to 01:55 DST. For example, if you're using the America/New_York timezone (Eastern time), you would:

a. Set the timezone to GMT.

b. Use 'date' to set the time to 05:55 on Nov. 4. (05:55 GMT = 01:55 EDT; adjust as needed for your timezone if not Eastern).

c. Set the timezone back to America/New_York.

d. Use 'date' to verify that the time displays as 01:55 EDT, not 01:55 EST.

e. Wait five minutes.

Hope this helps,
I work for HP
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Joseph P. Smith
Regular Advisor

Re: DST and NTP Interaction


Thanks for the correction on the "fall back" phase of testing. That is very helpful.

According to /etc/services and /etc/inetd.conf there's no entry for ntpd/xntpd, so it's a once a day synch with the timeserver.

Joe S.