Introduction

Windows encodes the time zone transition dates on a yearly basis yielding the following rules for years 2006, 2007, and 2008:

DST End

DST Start

2006

Third Sunday in March

First Sunday in October

2007

Third Sunday in March

Last Sunday in September

2008

First Sunday in April

Last Sunday in September

The definition of the New Zealand time zone differs between Windows Vista and down-level Windows OSes.   Specifically, the difference between OSes relates to which time zone rule each OS returns as the currently active rule; Vista returns the 2007 rule shown in the table above while other Windows OSes return the 2008 rule.

This difference exists because Vista is designed to keep the 2007 rule as the active rule until December 31st, at 11:59:59PM.  At midnight, January 1st , Vista will automatically activate the 2008 rule.  Previous OSes such as Windows XP and Windows Server 2003 do not have the capability to automatically switch rules, so after the time zone patch under KB 933360 is applied; they make the 2008 rule active. 

This inconsistency causes user-visible differences in the functioning of the Outlook Calendar on Vista versus non-Vista OSes.  The below is intended to explain the end-user visible affects as well as provide some guidance on how to best plan for rebasing calendars in this region.

Example

In the following example, two weekday recurring appointments were created on fully patched OSes.  Both computers had the current time zone set to New Zealand and both created the appointments such that they occurred from 12:00pm to 12:30pm local time.  In the screen shots below, the green series was created on a down-level OS (and, therefore, encoded using the 2008 time zone rules for New Zealand) while the red series was created on Vista (and, therefore, encoded using the 2007 time zone rules for New Zealand).

As can be seen in the table above, this means that the two recurrences differ by how they were encoded between the third Sunday in March and the first Sunday in April.  To illustrate the issue, let us now display the items on the two different machines:
 

Down-level OS

On the down-level OS, the 2008 New Zealand rule is in force and, therefore, daylight savings time ends on April 6th.  This means that the item encoded in 2008 rules continues to occur at noon but the item encoded with 2007 rules now appears an hour too late since it was encoded under the expectation that daylight savings time is no longer in force.

 

The discrepancy continues until we get to the first Sunday in April, at which point both appointments return to their intended local time.

 

Windows Vista

When viewed on Vista, exactly the same appointments in the same store look different.  Again, everything agrees except for the “delta” between the third Sunday in March and the first Sunday in April only, on Vista, it is the item encoded with 2007 rules that continues to look correct while the item encoded in 2008 rules now looks an hour early because it was encoded under the expectation that daylight savings time would continue and the Calendar is being displayed as if it does not.

 

As above, the discrepancy continues until we get to the first Sunday in April, at which point both appointments return to their intended local time.

 

Outlook Calendar Rebasing

The Time Zone Data Update Tool for Microsoft Office Outlook is aware of the above discrepancy and attempts to handle it as correctly as possible.  Specifically, the client-side tool will detect the OS on which it is running and behave as expected on the assumption that all appointments were created on that OS.  Some examples (assuming a fully patched machine in all cases):

• When run on Vista during 2007 with the time zone set to New Zealand, the tool will assume that any appointments that lack time zone information were created using the 2006 rules and will transition all appointments to the 2007 rules.
• When run on Vista during 2008 with the time zone set to New Zealand, the tool will assume that any appointments that lack time zone information were created using the 2007 rules and will transition all appointments to the 2008 rules.
• When run on a down-level OS in either 2007 or 2008, the tool will assume that any appointments that lack time zone information were created using 2006 rules and will transition them directly to the 2008 rules.
It is worth noting that, in all versions of Outlook, all recurrences are stamped with time zone information and, therefore, there is no ambiguity in behaviour.  Single instance items, on the other hand, are not stamped with time zone information on Outlook 2003 SP2 and earlier so, in such cases, care will have to be taken to review the times of all single-instance appointments to ensure that the updated calendar items are correct.

All of the behaviour described above is specific to the latest version of the Time Zone Data Update Tool for Microsoft Office Outlook and the Exchange Calendar Update Configuration Tool.  Previous versions of both tools do not behave properly in time zones such as New Zealand so users/administrators should be sure to have the latest versions of the appropriate rebasing tool before starting.  The latest versions of the Time Zone Data Update Tool for Microsoft Office Outlook and the Exchange Calendar Update Configuration Tool along with supporting documentation can be found at http://support.microsoft.com/kb/931667 and http://support.microsoft.com/kb/941018 respectively.

Mitigations

Here are some things to think about as you consider your plans for rebasing Calendars in the New Zealand time zone:
• It is beneficial to rebase a calendar on the OS from which the majority of single instance items were created (although care still needs to be taken to ensure that the correct updates were applied).
• In the case of Vista, users will have to rebase their Calendar a second time in January of 2008.  It is recommended that this rebasing be done as early in 2008 as possible so as to avoid user confusion as late March and early April approach.
• If using the Exchange Calendar Update Configuration Tool, the above logic applies to the machine from which the Exchange Calendar Update Configuration Tool was run (i.e. that machine should be chosen such that it matches the most common OS deployed across the company and a second rebasing will have to be scheduled if users create appointments using Vista).
 

Conclusion

There are three end-user visible side-effects to the interaction of the Outlook Calendar and the definition of the New Zealand time zone on various Windows OSes:
1. When looking at appointments in late March or early April, there will be a discrepancy between the local time at which the appointments appear to start and end.
2. Users that only use down-level OSes to create/edit items on their Calendar need to rebase only once, during the fall of 2007.
3. Users that use Vista to create/edit items on their Calendar will need to rebase twice: once in the fall of 2007 and again in January of 2008 in order to transition any items encoded using the 2007 time zone rules into the 2008 time zone rules.  Once this step is successfully completed, anomalies such as the above should simply disappear.

We greatly appreciate the assistance and feedback that Graham Codd provided on this issue.

Nick.