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