The Old New Thing

Time - The Old New Thing

Why is the Win32 epoch January 1, 1601?

March 6, 2009 Mar 6, 2009 03/6/09
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...

Working with ambiguous and invalid points in time in managed code

October 31, 2008 Oct 31, 2008 10/31/08
Raymond Chen
Public Service Announcement: Daylight Saving Time ends in most parts of the United States this weekend. I pointed out some time ago that Win32 and .NET deal with daylight saving time differently. Specifically, Win32 always deals with the time zone you are currently in (even if it's not the time zone that corresponds to the timestamp you are...

Why can't I convert a time zone abbreviation into a time zone?

March 7, 2008 Mar 7, 2008 03/7/08
Raymond Chen
Suppose you have a string of the form"Thu Mar 27 03:46:20 CST 2003".How can you parse this into something your program can manipulate,like say a SYSTEMTIME or a FILETIME?Basically, you can't in the general case.The time zone abbreviation CST is ambiguous.It could meanU.S./Canada Central Standard Time,Australian Central Standard ...

Don't be so fast to discount those oddball time zones

March 9, 2007 Mar 9, 2007 03/9/07
Raymond Chen
This weekend marks the beginning of Daylight Saving Time in most parts of the United States, the first year under the new transition rules in the Energy Policy Act of 2005. Pay extra attention to your clocks this weekend. If you have a device that automatically adjusts for Daylight Saving Time, and it hasn't been updated for the new ...

How to recognize different types of sentinel timestamps from quite a long way away

October 28, 2005 Oct 28, 2005 10/28/05
Raymond Chen
Some time ago, I discussed several timestamp formats you might run into. Today we'll take a logical step from that information and develop a list of special values you might encounter. Note that if you apply time zone adjustments, the actual timestamp may shift by up to a day. All of these special values have one thing in common: If you see...

Why can’t you treat a FILETIME as an __int64?

August 25, 2004 Aug 25, 2004 08/25/04
Raymond Chen
The FILETIME structure represents a 64-bit value in two parts:typedef struct _FILETIME { DWORD dwLowDateTime; DWORD dwHighDateTime;} FILETIME, *PFILETIME;You may be tempted to take the entire FILETIME structureand access it directlyas if it were an __int64.After all, its memory layout exactly matchesthat of a 64-bit (little-...

Why do timestamps change when I copy files to a floppy?

February 26, 2004 Feb 26, 2004 02/26/04
Raymond Chen
Floppy disks use the FAT filesystem, as do DOS-based and Windows 95-based operating systems. On the other hand, Windows NT-based systems (Windows 2000, XP, 2003, ...) tend to use the NTFS filesystem. (Although you can format a drive as FAT on Windows NT-based systems, it is not the default option.) The NTFS and FAT filesystems store ...