Daylight Saving Time

SUMMARY

Starting in 2007 the start and end dates for Daylight Saving Time in the United States changed.  This causes problems for HCA if your installation of Windows doesn't correctly know that the clock should or shouldn't be showing Daylight of Standard Time.  This technical note explains what HCA needs for correct calculations and suggestions one method to have Windows understand the new start and end dates.

Sunrise and Sunset in HCA are computed from the location you specified for your home (latitude and longitude), the time zone you specified in Windows in the Date-Time control panel applet, and if the clock is currently showing Daylight Saving Time or not.

As you know, the dates for the start and end date for Daylight Saving Time changed. Unless you keep Windows updated with all the various fixes that Microsoft supplies, your computer clock may not be showing Daylight Saving Time today as it should.

It is important that you understand that both the clock must be correct and the internal data in Windows that says Daylight Saving Time is in effect for the sunrise and sunset calculations to be correct. It isn't good enough to just change the time on your computer to be correct.

To check, open the control panel and then start the date-time applet - you can also do this by a double-click on the time shown in the Task Bar. If on the "Date & Time" tab it doesn't show "... Daylight Time" HCA will not get the sunrise and sunset calculations correct.

There are lots of ways to get Windows to understand the new start and stop dates rules for Daylight Saving Time. Some require service packs and downloading Windows updates or require using the system registry editor. There is a much simpler method described here:

http://support.microsoft.com/kb/914387

Ignore most of this article! About 3/4 way though the page you will see a section titled "Method 2: Change the time zone settings on a single computer". It describes how to use a program called TZEdit which is a very small download from the Windows site. The article has a link to it.

Using the program is very easy. You can change all the US time zones - or just the one you are in - to have the new start and stop rules. Just follow the instructions in the article. The new rules are:

Note: Follow the instructions in the article after the edits to change to a different time zone and then back to the correct one in order for Windows to see the changes. You don't have to restart your computer.

If you are using Windows 98, Me, or Windows 2000 you need a different fix.  A good one is available from http://www.intelliadmin.com/blog/2007/02/windows-98-me-dst-patch.html

After making the time zone fix also check that:

1. Your computer clock is correct
2.  The date time applet is showing Daylight Time
3.  The time zone selected matches the latitude an longitude that HCA has for your home.

Once you have completed the changes to get the Daylight Saving Time correct on your computer, and you have the correct time and time zone selected, and your latitude and longitude in your HCA design matches that time zone, then sunrise and sunset should be correct in HCA. You can check that by selecting from the menu: HCA - Status.