Date to Windows FILETIME
Converts a date into a Windows FILETIME — 100-nanosecond intervals since 1 January 1601 UTC. This is the value in file timestamps, the registry and Active Directory.
How to use the date to windows filetime
The 1601 epoch looks arbitrary and is not. The Gregorian calendar repeats on a 400-year cycle, and 1601 is the start of the first full cycle after the calendar was adopted in 1582 — choosing it means the day-of-week and leap-year arithmetic is a clean cycle from the epoch rather than a special case. It also comfortably predates any date a computer would need to store.
FILETIME shares its 100-nanosecond unit with .NET ticks but not its epoch, which is a genuinely dangerous similarity: two large numbers in the same unit, 504 years apart. Confusing them produces a date that is wrong by exactly 1,600 years, and the offset between them is 504,911,232,000,000,000.
Where you meet this is Active Directory, and it has a quirk worth knowing: accountExpires and lastLogonTimestamp use FILETIME, but the value 0 and the value 9223372036854775807 both mean "never" rather than an actual date. Converting either literally gives 1601 or the year 30828, and code that does not special-case them produces exactly those nonsense values in reports.
Questions
It is the start of the first full 400-year Gregorian cycle after 1582, which makes the calendar arithmetic clean.