Wednesday, 8 August 2007

Daylight Saving 2: Appointments that jump without moving

In my previous post I talked about how Outlook stores appointment times in UTC (time-zone neutral) and converts them from/to your local time zone when you create or view an appointment.

I briefly hinted, right at the end, at the fact that some countries like India (and most of Asia and the Far East, amongst others) don't put their clocks forward and back each spring and autumn. This can cause confusion when scheduling conference calls across regions and continents, when some participants observe daylight saving and others don't.

Today I'll take this further with a worked example:

Imagine Bilal works in Mumbai, and books a regular weekly meeting for his call each Monday with Zoe in London. Mumbai is 5½ hours ahead of GMT, and does not do daylight saving. London, like most of Europe, does observe daylight saving. Through the winter months, the meeting is at 4:00pm for Bilal and 10:30am for Zoe. In the Spring, because London goes to Daylight Saving Time (but Mumbai does not), the meeting 'moves' to 9:30am for Zoe, but stays at 4:00pm for Bilal.

Winter (booked from Mumbai, viewed in London)

Local time in Mumbai

16:00

UTC

10:30

Remove GMT+5.5 time zone

-5:30

Same as GMT time zone

+0:00

No daylight saving

-0:00

No daylight saving

+0:00

UTC

10:30

Local time in London

10:30

Summer (booked from Mumbai, viewed in London)

Local time in Mumbai

16:00

UTC

10:30

Remove GMT+5.5 time zone

-5:30

Same as GMT time zone

+0:00

No daylight saving

-0:00

Add daylight saving

+1:00

UTC

10:30

Local time in London

11:30

This causes a problem for Zoe because she has another meeting at 9:30am on Mondays. Thankfully, Bilal's calendar is less congested on a Monday afternoon, so the solution is simple; they agree that the meeting should be scheduled from Zoe's time zone. Bilal cancels the appointment and Zoe sets up a new one for 10:30am London time. Now the appointment appears at 5:00pm for Bilal, but in the autumn it will 'move' to 4:00pm, while staying at 10:30am for Zoe.

Winter (booked from London, viewed in Mumbai)

Local time in London

10:30

UTC

10:30

Already in GMT time zone

+0:00

Adjust to GMT+5.5 time zone

+5:30

No daylight saving

-0:00

No daylight saving

+0:00

UTC

10:30

Local time in Mumbai

16:00

Summer (booked from London, viewed in Mumbai)

Local time in London

10:30

UTC

09:30

Already in GMT time zone

+0:00

Adjust to GMT+5.5 time zone

+5:30

Remove daylight saving hour

-1:00

No daylight saving

+0:00

UTC

09:30

Local time in Mumbai

15:00

This has taken a lot more words than I thought it would, so I'll save the story of how this changed this year for next time...

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Creative Commons License This work by TechieBird is licensed under a Creative Commons Attribution-No Derivative Works 2.0 UK: England & Wales License.