March 6th, 2009

Why is the Win32 epoch January 1, 1601?

Raymond Chen
Raymond Chen

Public Service Announcement: This weekend marks the start of Daylight Saving Time in most parts of the United States.

The FILETIME structure records time in the form of 100-nanosecond intervals since January 1, 1601. Why was that date chosen?

The Gregorian calendar operates on a 400-year cycle, and 1601 is the first year of the cycle that was active at the time Windows NT was being designed. In other words, it was chosen to make the math come out nicely.

I actually have the email from Dave Cutler confirming this.

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Old New Thing
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Raymond Chen

Raymond has been involved in the evolution of Windows for more than 30 years. In 2003, he began a Web site known as The Old New Thing which has grown in popularity far beyond his wildest imagination, a development which still gives him the heebie-jeebies. The Web site spawned a book, coincidentally also titled The Old New Thing (Addison Wesley 2007). He occasionally appears on the Windows Dev Docs Twitter account to tell stories which convey no useful information.

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