Notes About Time In Windows

How Windows stores time.
  There are two binary types. One is the count of 100 ns periods in two DWORDs, called FileTime. The other is a number for the year, month, day of month, hour, minute, second and milliseconds each in a WORD (unsigned 16 bit type) called SystemTime. It also has a WORD for the day-of-week ID.

Range of FileTime and SystemTime.
  The "epoch" for Windows NT family of operating systems (OSs) is 1 January 1601^{1, 2 and 3} and ranges to the maximum/latest/largest for each type. Zero in FileTime is midnight on 1 January 1601. The first/earliest SystemTime is 1601, 01, 01, 0, 0, 0 and 0. For input to a SystemTime, do not use day-of-week. That will take the next discussion to describe.

Largest/Latest Date-Times for FileTime and SystemTime
  In a FileTime bit 63 is never "set". That would indicate a negative value in a signed type. Microsoft mentions __int64* (a signed type) on their FileTime page¹. So, the maximum value for a FileTime is 7FFFFFFFFFFFFFFF₁₆. That value plus one causes FileTimeToSystemTime ¹ to error. (Source code for a PB Demo" of maximum FileTime and SystemTime.) That represents 30828-09-14T02:48:05.4775807Z (in ISO 8601 format) The latest date where function SystemTimeToFileTime² does not fail is: 30827-12-31T23:59:59.999Z or 7FFF35F4F06C58F0₁₆ for the equivalent FileTime value. Since 30827-12-31T23:59:59.999Z works in both directions, I'll call that the maximum "usable" Date-Time for Windows. (30827 - 2023 is 28804 years. There is nothing to worry about any time soon. 😄 )

Use of QUAD instead of FileTime in PB.
  Microsoft recommends against doing arithmatic operations on variables of type FileTime¹ Also, "Do not cast a FILETIME pointer to either a ULARGE_INTEGER* or __int64* value because it can cause alignment faults on 64-bit Windows."; __int64* is C++ equivelent of PB's QUAD. (PB compiled programs operate in 32 bit environment in 64 bit Windows, but why form bad habits?) I dislike the extra step of copying as Microsoft suggests. They do not say anything about the rerverse, using a pointer of a QUAD with API functions that need a FILETIME pointer. A way to do this is a UNION of QUAD and FileTime; PB includes in \WinAPI\ directory has one defined. The "AS FILETIME" in a function call could be changed to "AS QUAD" in the include files, or a copy of the DECLARE in the source code, or by putting "BYVAL VARPTR(varname AS DWORD" in the call statement.

Resolution of FileTime and SystemTime.
  Even though a FileTime, or its timer are used for many functions, the resolution is in the milliseconds. Even though the low bits eventually change in a FileTime variable, it is updated in "steps" of multiple milliseconds. Raymond Chen⁴ (see footnote 4) explains it well. The times used in the file system (CreationTime, LastAccessTime and LastWriteTime) are a second or worse. GetSystemTime and the SystemTime type only resolve down to milliseconds or multiple milliseconds. If you use GetSystemTimePreciseAsFileTime, then FileTimeToSystemTime, ignore the milliseconds member, do MOD 10000000₁₀ on the FileTime, and append the result you get (upto) a 100₁₀ ns resolution time stamp string. (method shown at end of PB Demo" source code)

PowerBASIC Time Related Functions / Statements.

DATE$ and TIME$

Okay if used individually for a STRING. If used together to make a date/time stamp, a rare but real ambiguity can cause a 24 hour error. For example, if DATE$ gets executed a few nanoseconds before midnight and TIME$ a nanoseconds after midnight, the time returned will look like it is for what is now yeesterday.

DAYNAME$

Will work with wDayOfWeek number from a SystemTime variable^{2, 5}. Other sources may be 1 to 7 for Sunday to Saturday, or 1 for Monday, etcetera.⁵

MONTHNAME$

Will work with wMonth number from a SystemTime variable.

SLEEP

Very low resolution which also has variable resolution depending on how other running programs set the counter it uses. To just stop program execution for "at least" some rough time, it works. For an "at least" timer put SLEEP in a thread. For more accuracy in either case use a WaitableTimer.
SLEEP 0 is not any kind of timer. It "breaks up" a tight loop, or gives back unneeded time slice time to the system.

TIMER

The number of seconds since midnight. 0.01 s resolution in a floating point variable that can not hold tenths, hundredths, etcetera exactly.
Anybody actually use this for anything?

TIX

By the wording for TIX in PB Help, content of "What does QueryThreadCycleTime actually count?"⁶ may apply. The interval timer at High Resolution Interval Counter would be better to compare code speed.

PowerTime Collection (Objects and Methods)

Uses QUAD variables to hold FileTime defined value (discussed above).

Created on 19 March 2023

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