NTP / Daylight Saving

  pauldonovan 10:58 27 Apr 2005


Bit of an interesting one. If you have servers set to automatically update their time from a time server using NTP (Network Time Protocol?), what actually happens when there is a change in the time e.g. daylight saving.

Say the clock goes back 1 hour. Does the clock on the servers go through to 1am then go back to midnight?! Doesn't that mean that from the server perspective there will actually be 2 occurences of each time between midnight and 1 i.e. any application logging date/time or any scheduling application will be confused by this?!

Thanks in advance,


  mgmcc 15:11 27 Apr 2005

The "actual" time is Greenwich Mean Time with time shifted from that depending on the time zone you are in. Time itself doesn't change, only your clock changes if you decide you want to go one hour ahead in the summer, therefore a time server doesn't need to change.

Individual PCs which are set to adjust for "Daylight Savings Time" move their clock forward by an hour from GMT to BST but "actual" time indicated by a time server remains unchanged and still related to GMT.

  pauldonovan 15:30 27 Apr 2005

mgmcc, I need to clarify something though...it wasn't so much the time server I was concerned with it was other servers that are configured to update their time from the time server.

What happens to them in the change over?

  mgmcc 15:50 27 Apr 2005

I think you've lost me.

Actual time and the "Time Server" stays constant.

"Network Servers" update via the time server but adjust their own clocks for Daylight Savings Time.

"Network Clients" update via the time server but adjust their own clocks for Daylight Savings Time.

Therefore the "Network Servers" and "Network Clients" should remain "time synchronised".

...as you say though, when Daylight Saving Time ends in the Autumn and the clocks go back, there will be two instances of the hour immediately prior to the change, one being BST based and the other GMT based. In the Spring, an hour is missing.

  pauldonovan 16:59 27 Apr 2005

...sorry to have lost you, it was your last point I was looking for clarification on.

When Network Servers/Network Clients update during Daylight Saving time they will effectively see two instances of an hour. Which plays havoc with scheduling/logging systems I would have thought.

We have scheduled tasks and I've steered clear of scheduling them for 1am because I'm worried that during Daylight Saving they'll run twice or something!

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