What is the significance of January 1, 1601?

Because 1/1/1601 was the start of the epoch.

Take it from Raymond Chen:

Why is the Win32 epoch January 1, 1601?^🕗

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.

Bonus Chatter

RFC4122 UUIDs also measure 100ns ticks, but they start at 10/15/1582 (as opposed to FILETIME's 1/1/1601:

Date                    Ticks               Uuid Epoch ticks
----------------------  ------------------  ------------------
1582-10-15              -5748192000000000   0                    Start of uuid epoch
1601-01-01              0                   0x00146BF33E42C000   Start of Windows epoch
1899-12-30              0x014F35A9A90CC000  0x0163A19CE74F8000   Lotus 123/Excel/Access/COM zero date
1900-01-01              0x014F373BFDE04000  0x0163A32F3C230000   SQL Server zero date
1970-01-01              0x019DB1DED53E8000  0x01B21DD213814000   Unix epoch timestamp
2000-01-01              0x01BF53EB256D4000  0x01D3BFDE63B00000
2010-01-01              0x01CA8A755C6E0000  0x01DEF6689AB0C000
2020-01-01              0x01D5C03669050000  0x01EA2C29A747C000

//FILETIME eras
1972-01-21 11:43:51 PM  0x01A0000000000000  0x01B46BF33E42C000   Start of 0x01A era
1986-04-30 11:43:13 AM  0x01B0000000000000  0x01C46BF33E42C000   Start of 0x01B era
2000-08-06 11:42:36 PM  0x01C0000000000000  0x01D46BF33E42C000   Start of 0x01C era
2014-11-14 11:41:59 AM  0x01D0000000000000  0x01E46BF33E42C000   Start of 0x01D era
2029-02-20 11:41:22 PM  0x01E0000000000000  0x01F46BF33E42C000   Start of 0x01E era
2043-05-31 11:40:44 AM  0x01F0000000000000  0x02046BF33E42C000   Start of 0x01F era

//UUID eras
1968-02-11 11:43:13 AM  0x019B940CC1BD4000  0x01B0000000000000   Start of uuid 0x01B era
1982-05-20 11:42:36 PM  0x01AB940CC1BD4000  0x01C0000000000000   Start of uuid 0x01C era
1996-08-27 11:41:59 AM  0x01BB940CC1BD4000  0x01D0000000000000   Start of uuid 0x01D era
2010-12-04 11:41:22 PM  0x01CB940CC1BD4000  0x01E0000000000000   Start of uuid 0x01E era
2025-03-13 11:40:44 AM  0x01DB940CC1BD4000  0x01F0000000000000   Start of uuid 0x01F era

Bonus Chatter

Excel uses a zero date of 12/30/1899 in order to be bug-for-bug compatible with Lotus 1-2-3. Which is also why Excel considers February 1900 to be a leap year (because the Lotus 1-2-3 guys thought it was). Which is why it's also impossible to represent dates before March 1, 1900 in Excel.

Well, 1 January 1601 was the first day of the 17th Century. And pendulum clocks were invented in the 17th century, allowing time to be measured to 1 second accuracy¹. So (in theory) there might be references in extant literature from that period to timepoints measured with that accuracy.

But in reality the choice is arbitrary. There has to be an "epoch", and provided

1. the epoch is far enough back that "negative time" values are rare, and

2. the wrap-around time is far enough in the future to be a few generations away,

any choice will do.

But hey, if it worries you that much, send a letter to Steve Balmer².

I'm inclined to believe Ian Boyd's answer, given the claimed source. And the reason therein is that it makes the math easier (for Gregorian leap year calculation). However, given how tiny that simplification is, and how weak the reasoning behind it, the choice is (IMO) essentially arbitrary. (Not that I'm saying it is wrong ...)

^{1 - OK ... probably not that accurate.}

^{2 - Or Satya Nadella.}

Its a pragmatic choice.

The modern western calendar was not consistent until 1752 when Britain (and its colonies) adopted the Gregorian calendar, which had been adopted in most of catholic Europe since 1582.

This is the modern calendar with leap years etc. to keep the 1st of January aligned with the winter solstice.

So why not start from 1st January 1752? Because the basic leap year rule "Its a leap year if the two digit year is divisible by four except if the four digit century is also divisible by four") established a 400 year cycle. The first full cycle starting on 1st January 1601, (at least in Rome).

The leap year and date calculations are painful enough without starting midway through a four hundred year cycle so 1600 is a pretty good start as long as you remeber that any dates before 1752 need to be qualified by a geographic location, as British dates were 10 days out of sync. with Roman dates by this time.

As has already been mentioned I think the popular answer is because 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.

January 1, 1601 is origin of COBOL integer dates.

It is also day 1 by ANSI date format.

And if you speculate further according to ISO8601 which is the format in which it is in, prior to 1583 time was based on the proleptic Gregorian calendar which has 366 days per year. Perhaps they just rounded up to the next century.