Why Can’t The Windows Registry Timezones be used?

A poster with the handle iwonder started a thread on the ASP.net forum, titled On Time Zones and Daylight Saving Time (Summer Time).. (I've added the whole thread to the "Cautionary Tales" section of the Chronos web site.) In a subsequent post to the thread, he has a subsection titled "Why Can’t The Windows Registry Timezones be used?", where he states:

First, and foremost the 75 windows registry timezones only are setup to support current year. Meaning that there is no historical reference. In fact, when a given locale changes their time zone daylight saving (summer time) rules, these rules need to be manually ‘fixed’ in the registry. These types of corrections are not distributed by Microsoft as updates to anyone. The corrections appear in KB articles, but the corrections could easily be missed by folks that are not in the locale affected. As each year starts, new rules are in place, which may not calculate the correct date time adjustments for any past years. Actually, any date time adjustment based on the registry alone results in a date time factored with the current years’ rules. Not exactly the result time sensitive sites would require.

Secondly, the windows registry is not totally accurate. Independent tests show it to be around 75-80% accurate at any point in time. Also, the time zones do not represent every time zone on the globe, just the more commonly used zones are included. The UNIX / Linux TZInfo database includes many more time zone descriptions, but there is a question of just how many require support. Obviously, MS made the determination that only 75 of the more than 108 zones in the TZInfo database really require support. It’s a question of judgement.

Lastly, web applications are not really meant to have access to the windows registry. It’s really a question of security, but I can’t imagine any host provider wanting to allow a web application to manipulate the registry. In fact, I’m not sure if the Code Access Security model will even allow the type of registry read and write permission required to effectively use and maintain a registry timezone information.

Correct answers in date/time computations are just as important as correct answers in issuing payroll checks, crediting interest earned, sending out invoices, computing work schedules, sheduling airline flights, administering the right medications to the right patients at the right time, getting a spaceship, rocket or missile to its intended destination at the right time, or even just dividing one number by another.

When I hear people say otherwise, there's no doubt in my mind that what I'm really hearing is an emotional reaction against the unwanted complexity of the date/time domain. But neither reality nor complexity goes away just because you wish things were otherwise.

Time is complex. Deal with it. The Universe doesn't care about your preferences, and will happily bite you in a private part of your anatomy if you can't bring yourself to accept and handle the world as it is.

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• Chronos blog home page
• Chronos Date/Time Library web site