Daylight Saving 1: A primer on appointment times

In my previous post I promised an explanation of how changes to Daylight Saving Time affect Outlook. But before we can even look at Daylight Saving, we need to understand how Outlook stores appointment times and copes with time zones.

Outlook stores each appointment with a start time and duration. (The end time isn't stored; Outlook works it out on-the-fly.) The start time is stored as UTC, calculated relative to the time zone of the machine the appointment is created on.

UTC stands for Universal Time Coordination – actually the same as GMT, but we refer to it as UTC to avoid confusion with "London Time" which is an hour off from 'real' GMT for half the year.

The easiest way to get to grips with this is through an example. Imagine Alice is in New York. She creates an appointment to remind her about her dentist appointment at 10am on a Wednesday in December. Outlook does this simple calculation based on the time zone of the computer Alice creates the appointment from:

Local time	10:00
Remove GMT-5 time zone	+5:00
UTC	15:00

So Alice's 10am appointment is actually stored at 15:00 UTC.

Two things to note: first, I deliberately chose December because daylight saving won't apply, and second, I'm using 24-hour clock times for simplicity.

When an appointment is viewed, Outlook converts the start time from UTC back to Local Time based on the time zone settings of the local computer.

So Alice views the appointment at her computer in New York, she'll see it as happening at 10am and never needs to know this calculation back and forth took place. But what if Alice were to view her calendar from another time zone?

Let's say Alice travels to Mumbai, and uses a local machine in the Mumbai office to view her Calendar, which she can do because it's stored on the Exchange server. India's Time Zone is GMT+5.5 so her dentist appointment will appear at the following time:

UTC (which we worked out already)	15:00
Adjust to GMT+5.5 time zone	+5.30
Local time in Mumbai	20:30

Nothing has changed in the way the meeting is stored, just the machine it's being viewed from has a different time zone so it appears with a different start time.

So now we'll add DST into the mix. Here's how the calculations would work if the appointment had been in June:

Local time in New York	10:00
Remove GMT-5 time zone	+5:00
Remove daylight saving hour	-1:00
UTC	14:00

so the appointment would be stored at 14:00 UTC

and the display time would be:

UTC (from above)	14:00
Adjust to GMT+5.5 time zone	+5.30
No adjustment for DST	0:00
Local time in Mumbai	19:30

(Some places don't do daylight saving at all. India is one of them.)

Next time I'll cover some of the things we see working across time zones which do DST differently.

Coming soon: 2007 Daylight Saving changes - The Sequel

I bought my quarterly train ticket just the other day. It expires in October. (I know that should be obvious, but we haven't had much sunshine here yet so it doesn't seem fair that autumn is so close.) That's depressing enough in itself, but I also just realised that the next round of Daylight Savings fun and games will soon be upon us.

For anyone who wasn't supporting Outlook a few months ago (or was in a country unaffected by the changes or just crawled under a rock and stayed there throughout March), the US government passed a piece of legislation in 2005 changing the dates the clocks "spring forward" and "fall back", starting from spring 2007. This gave three weeks more Daylight Saving time, and was calculated to save money and energy by people needing to use less heating and lighting.

Unfortunately, Windows only knows how to cope with one set of Daylight Saving Time (DST) rules per time zone. So, for instance, in London it knows we go into DST on the last Sunday in March, and we go back to GMT (Standard Time) on the last Sunday in October.

This might not sound like a problem, except that every time you schedule a recurring appointment in Outlook, it bases all future occurrences on what it knows about your time zone and DST settings now. So if those settings change, the meeting will appear to change. Which (as a lot of people found out) was exactly what happened.

This is a pretty complicated topic to explain. I spent the best part of three months this year talking about little else, and I still managed to get confused mid-presentation on more than one occasion! So for my own sanity and yours I'm going to stop now and leave the explanation of how Outlook saves appointments in UTC and displays them in local time for my next post.

How to Delete Outlook Temporary Files 2

In an earlier post, I gave instructions for clearing Outlook temporary files. Of course I forgot that the location of Temporary Internet Files in Windows Vista is different to XP/2000/NT, so different instructions are required. (Thanks to Oriolus for reminding me.) I also found that this is a lot less of a pain in Outlook 2007 than before.

(This procedure involves deleting files. The ones in the folder we're looking at are temporary files whose 'master' copy is held in the mailbox, so it doesn't result in loss of data. However, just in case someone should manage to mistype one of the commands and end up in the wrong folder, the usual disclaimer applies.)

For Outlook 2003 or earlier in Windows Vista:

1. Start > Run > Type "cmd" (without the quotes) > Click OK

In the command line, type the commands in bold (excluding my numbering and hitting return after each line)

2. cd "%localappdata%\microsoft\windows\temporary internet files"

3. dir /ad

You should now see a list of folder names - one or more will begin with OLK.

4. cd olk*

This should make the first OLK folder the current directory - type the full folder name instead of OLK* to select a different one)

5. explorer .

Note the space before the "."

This will bring up a Windows Explorer window showing the contents of the folder which you can then delete, but only after you've checked the address bar to make sure you're in the right folder!

If there was more than one OLK folder you can get to it by typing "cd.." at the command prompt then repeating steps 3 and 4 to select the OLK folder with the next folder name.

Type exit in your command window to close it, or just click on the x when you're done.

Maybe someone in the Outlook product group felt our pain, because in Outlook 2007 this has suddenly got a lot easier!

For Outlook 2007 in Windows XP:

1. Click Start > Run > Type "%userprofile%\local Settings\Temporary Internet Files\content.outlook" and click OK
2. In the Explorer window that appears, double-check you have the correct folder selected and delete its contents.

For Outlook 2007 in Windows Vista:

1. Click Start > All Programs > Accessories > Run* > Type "%localappdata%\microsoft\windows\Temporary Internet Files\content.outlook" and click OK *unless, like me, you've already added the Run command to your Start menu.
2. In the Explorer window that appears, double-check you have the correct folder selected and delete its contents.