function CenturyOfDate(const DateTime: TDateTime): Integer; 
function CenturyBaseYear(const DateTime: TDateTime): Integer;

Delphi Jedi Code Library, JCLDateTime.Pas unit

The CompareFileTime function compares two TFileTime values.

function HoursToMSecs(const Hours: Integer): Integer;
function MinutesToMSecs(const Minutes: Integer): Integer;
function SecondsToMSecs(const Seconds: Integer): Integer;

function TimeOfDateTimeToSeconds(const DateTime: TDateTime): Integer;
function TimeOfDateTimeToMSecs(const DateTime: TDateTime): Integer;

function DateTimeToLocalDateTime(const DateTime: TDateTime): TDateTime;
function DateTimeToDosDateTime(const DateTime: TDateTime): TDosDateTime;
function DateTimeToFileTime(const DateTime: TDateTime): TFileTime;
function DateTimeToSystemTime(const DateTime: TDateTime): TSystemTime; overload;

function LocalDateTimeToFileTime(const DateTime: TDateTime): FileTime;
function LocalDateTimeToDateTime(const DateTime: TDateTime): TDateTime;

function DosDateTimeToDateTime(const DosTime: TDosDateTime): TDateTime;
function DosDateTimeToFileTime(const DosTime: TDosDateTime): TFileTime; overload;
function DosDateTimeToSystemTime(const DosTime: TDosDateTime): TSystemTime;
function DosDateTimeToStr(const DateTime: Integer): string;

function FileTimeToDateTime(const FileTime: TFileTime): TDateTime;
function FileTimeToLocalDateTime(const FileTime: TFileTime): TDateTime;
function FileTimeToDosDateTime(const FileTime: TFileTime): TDosDateTime; overload;
function FileTimeToSystemTime(const FileTime: TFileTime): TSystemTime; overload;
function FileTimeToStr(const FileTime: TFileTime): string;

function SystemTimeToDosDateTime(const SystemTime: TSystemTime): TDosDateTime;
function SystemTimeToFileTime(const SystemTime: TSystemTime): TFileTime; overload;
function SystemTimeToStr(const SystemTime: TSystemTime): string;         

Constant used to convert from D1 TDateTimes to those used in D2 and later.

Convert TDateTime to a DOS date/time value.  The FileAge, FileGetDate, and FileSetDate routines operate on DOS date-and-time values, and the Time field of a TSearchRec used by the FindFirst and FindNext functions contains a DOS date-and-time value.

See FileSetDate example.
See AlterFileTimeStamp.TXT example.

function DateTimeToUnix(const AValue: TDateTime): Int64; 

function DayOfDate(const DateTime: TDateTime): Integer; 

Delphi Jedi Code Library, JCLDateTime.Pas unit

Compute the number of days since a known Monday, like Jan. 1, 1900:

DaysSince1900 :=
     TRUNC(EncodeDate(year,month,day)) -
     TRUNC(EncodeDate(1900,1,1)) + 1;

Use the MOD function to get the day of the week
(0=Sunday, 1=Monday, ..., 6=Saturday):

DayOfWeek := DaysSince1900 MOD 7;

Note:  ISO-8601 specifies Monday=1, Tuesday=2, ..., Sunday=7.
Therefore,  ISO8601day := (DayOfWeek+5) MOD 7 + 1;
(Thanks to Peter Haas for correcting this formula for me.
See Peter's info and unit at "Datum und Zeit - ISO 8601,"
http://www.pjh2.de/datetime/iso8601/index.php)

function DaysInMonth(const DateTime: TDateTime): Integer;

Delphi Jedi Code Library, JCLDateTime.Pas unit

ESB Consultancy Developers of Mathematical & Statistical Software and Delphi Software.

ESBDates™ v1.1- Large collection of Date & Time Routines to supplement those supplied by SysUtils. Includes TimeZone Calculations, Date Arithmetic, Easter Calculations, etc.  Includes Help File and Full Source.  www.esbconsult.com

http://delphi.icm.edu.pl/ftp/d20free/esbdates.zip

Method 1.  "Old" Way for Windows 95/98 -- FirstInstallDateTime

VAR
    Data      :  ARRAY[BYTE] OF CHAR;
    DataLength:  INTEGER;

    // Use to extract 4-byte integer from data
    DataValue :  INTEGER  ABSOLUTE Data; 
    Key       :  hKey;
    ValueType :  INTEGER;

// Open the registry key
IF   RegOpenKeyEx(HKEY_LOCAL_MACHINE,
     'SOFTWARE\Microsoft\Windows\CurrentVersion',
     0,0, Key) = ERROR_SUCCESS
THEN BEGIN
  DataLength := SizeOf(Data);
  RegQueryValueEx(Key, 'FirstInstallDateTime', 
                  NIL, @ValueType, @Data[0], @DataLength);
  // Sample result:  08/11/2000 08:26
  ShowMessage(FormatDateTime('mm/dd/yyyy hh:nn',
              FileDateToDateTime(DataValue)));

  // Close the registry key
  RegCloseKey(Key);
END;

Method 2.  "New" Way for Windows 95/98

USES
  Registry;

procedure TFormIDKey.ButtonDisplayClick(Sender: TObject);
  VAR
    KeyValue   :  DWORD;
    Registry   :  TRegistry;
...
  KeyValue := $210000;   // 1/1/1980

  Registry := TRegistry.Create;
  TRY
    Registry.RootKey := HKEY_LOCAL_MACHINE;
    IF   Registry.OpenKeyReadOnly(
         'SOFTWARE\Microsoft\Windows\CurrentVersion')
    THEN BEGIN
      IF   (Registry.GetDataType('FirstInstallDateTime') = 
            rdBinary) 
                AND
           (Registry.GetDataSize('FirstInstallDateTime') = 
            SizeOf(KeyValue) )
      THEN Registry.ReadBinaryData('FirstInstallDateTime',
                                   KeyValue, SizeOf(KeyValue))
    END
  FINALLY
    Registry.Free
  END;

  LabelDateTimeValue.Caption :=
    FormatDateTime('mm/dd/yyyy hh:nn:ss', 
    FileDateToDateTime(KeyValue));

Method 3.  Alternative for Windows NT -- InstallDate

  VAR
    InstallDate:  TDateTime;
    KeyValue   :  DWORD;
    Registry   :  TRegistry;  
. . .
  KeyValue := 0;   // 1/1/1980    

  Registry := TRegistry.Create;
  TRY
    Registry.RootKey := HKEY_LOCAL_MACHINE;
    IF   Registry.OpenKeyReadOnly(
         'SOFTWARE\Microsoft\Windows NT\CurrentVersion')
    THEN KeyValue := Registry.ReadInteger('InstallDate');
  FINALLY
    Registry.Free
  END;
  // Simiilar to UNIX conversion 
  InstallDate := EncodeDate(1970,1,1) + KeyValue/86400;
  LabelDateTimeValue.Caption := FormatDateTime('mm/dd/yyyy hh:nn:ss',
                                  InstallDate)

Get a file's date time stamp
www.preview.org/q/q1008.shtml

Peter Below's UseNet Post about setting CreatedDateTime of a file

Jamie Frater's UseNet Post with UnixTimeToDateTime by code by Philippe Ranger showing use of SystemTimeToTzSpecifiedLocalTime

Delphi 5 Developer's Guide, pp. 291-292

Jamie Frater's UseNet Post with UnixTimeToDateTime by code by Philippe Ranger showing use of SystemTimeToTzSpecifiedLocalTime

See other examples:  "Windows' Current Version Date/Time" under FileDateToDateTime

// Adapted from Robert Lee's UseNet Post
// Minimally invasive and accurate down to about 40 cycles.
// Requires Pentium or greater
FUNCTION GetCycleCount:  Int64;   // D4-D5
ASM
  DB 0FH
  DB 031H
END;

procedure TForm1.Button1Click(Sender: TObject);
  VAR
    Start:  Int64;    // D4-D5
    Stop :  Int64;
begin
  Start := GetCycleCount;
  Sleep(2000);   // 2000 milliseconds = 2 seconds
  Stop := GetCycleCount;
  ShowMessage( IntToStr(Stop-Start) )
end;

Results on Pentiums:
120 MHz  240112244
166 MHz  332354144
450 MHz  899102646 

Based on these values, there are approx 1000141 counts/MHz.  This suggests the following function to estimate speed of processor:

FUNCTION ApproxPentiumSpeed:  INTEGER;
  VAR
    Start:  Int64;    // D4-D5
    Stop :  Int64;
begin
  Start := GetCycleCount;
  Sleep(1000);   // 1000 milliseconds = 1 second
  Stop := GetCycleCount;
  RESULT := ROUND( (Stop-Start) / 1000141)
END;

Also see GetTickCount and QueryPerformanceCounter

Ian Huang's UseNet Post with sample code

GetLocaleChar(DefaultLCID,LOCALE_IFIRSTDAYOFWEEK,'0')

See GetWinLocaleWeekDef function in DTWeeks.pas unit by Peter Haas.
http://www.pjh2.de/datetime/weeknumber/index.php
http://delphi.pjh2.de/units/download/DTWeeks.zip

Peter's examples show how to use GetLocaleChar values for LOCALE_IFIRSTDAYOFWEEK and LOCALE_IFIRSTWEEKOFYEAR, e.g.,

GetLocaleChar(DefaultLCID,LOCALE_IFIRSTDAYOFWEEK,'0')

See GetWinLocaleWeekDef function in DTWeeks.pas unit by Peter Haas.
http://www.pjh2.de/datetime/weeknumber/index.php
http://delphi.pjh2.de/units/download/DTWeeks.zip

Delphi 5 Developer's Guide, pp. 291-292

Windows 95/98 returns FALSE.

VAR
  Start:  DWORD;    // D2-D5
  Stop :  DWORD;
...
Start := GetTickCount;
Sleep(2000);   // 2000 milliseconds = 2 seconds
Stop := GetTickCount;
ShowMessage( 
  IntToStr(Stop-Start) + ' ' +
  Format( '%.3f', [ (Stop-Start)/1000 ])  + ' seconds')

Sample Output:  2001  2.001 seconds

The GetTickCount function retrieves the number of milliseconds that have elapsed since Windows was started. The internal timer wraps around to zero if Windows is run continuously for approximately 49.7 days. (Editorial:  Can Windows run that long without crashing?  Take a look at MS's Computer Hangs After 49.7 Days.)

Borland's "Timed beeps from the PC speaker," FAQ 1838D

Also, see QueryPerformanceCounter and GetCycleCount

Erez Steinberg's UseNet Post about using GetTimeZoneInformation

Read and translate the info in a TTimeZoneInformation record:
http://delphi.pjh2.de/units/download/TZView.zip

PROCEDURE TFormSystemInfo.ShowDateTime;
  VAR
    TimeZoneInformation: TTimeZoneInformation;

  FUNCTION DayInMonth(CONST w: WORD): STRING;
  BEGIN
    CASE w OF
      1: RESULT := '1st';
      2: RESULT := '2nd';
      3: RESULT := '3rd';
      4: RESULT := '4th';
      5: RESULT := 'Last';
      ELSE RESULT := 'Unknown'
    END
  END {DayInMonth};

BEGIN
  MemoInfo.Lines.Add('Date/Time');
  MemoInfo.Lines.Add(' Current Time ' +
  FormatDateTime('mm/dd/yyyy hh:nn:ss', Now));

  // See pp. 700-702 of "Tomes of Delphi 3: Win32 Core API."
  GetTimeZoneInformation(TimeZoneInformation);

  WITH TimeZoneInformation DO
  BEGIN
    IF   StandardDate.wYear = 0
    THEN MemoInfo.Lines.Add(' ' + StandardName + ' Starts ' +
         DayInMonth(StandardDate.wDay) + ' ' +
         LongDayNames[1+StandardDate.wDayofWeek] + ' of ' +
         LongMonthNames[StandardDate.wMonth])
    ELSE MemoInfo.Lines.Add(' ' + StandardName + ' Starts ' +
         Format('%2.2d/%2.2d/%4.4d %2.2d:%2.2d:%2.2d',
         [StandardDate.wMonth, StandardDate.wDay,
         StandardDate.wYear,
         StandardDate.wHour, StandardDate.wMinute,
         StandardDate.wSecond]))
  END;

  WITH TimeZoneInformation DO
  BEGIN
    IF   DaylightDate.wYear = 0
    THEN MemoInfo.Lines.Add(' ' + DaylightName + ' Starts ' +
         DayInMonth(DaylightDate.wDay) + ' ' +
         LongDayNames[1+DaylightDate.wDayofWeek] + ' of ' +
         LongMonthNames[DaylightDate.wMonth])
    ELSE MemoInfo.Lines.Add(' ' + DaylightName + ' Starts ' +
         Format('%2.2d/%2.2d/%4.4 %2.2d:%2.2d:%2.2d',
         [DaylightDate.wMonth, DaylightDate.wDay,
         DaylightDate.wYear,
         DaylightDate.wHour, DaylightDate.wMinute,
         DaylightDate.wSecond]));
  END;
END {ShowDateTime};

Getting a list of valid time zones from Windows, Borland's FAQ 2700D
Using GetTimeZoneInformation function, FAQ 2630D

IncMonth returns Date shifted by the specified number of months.
NumberOfMonths parameter can be negative, to return a date N months ago.  If the input day of month is greater than the last day of the resulting month, the day is set to the last day of the resulting month.  Input time of day is copied to the DateTime result.

Function that identifies whether specified year is a leap year.

function IsLeapYear(const Year: Integer): Boolean;  overload;
function IsLeapYear(const DateTime: TDateTime): Boolean;  overload; 

Delphi Jedi Code Library, JCLDateTime.Pas unit

Alternate Solution:

// [Meeus91, p. 65]
// DayOfYear(1978, 11, 14) = 318
// DayOfYear(2000, 11, 14) = 319
FUNCTION DayOfYear(CONST Year, Month, Day: WORD): INTEGER;
  VAR k: WORD;
BEGIN
  IF   IsLeapYear(Year)
  THEN k := 1
  ELSE k := 2;
  RESULT := TRUNC(275*Month/9) - 
                        k*TRUNC( (Month+9)/12 ) + Day - 30
END {DayOfYear};

Usage:
ShowMessage( FormatDateTime('mmmm dd, yyyy',
     LastDayInMonth(SpinEditYear.Value, SpinEditMonth.Value) ));

function MonthOfDate(const DateTime: TDateTime): Integer;  

Delphi Jedi Code Library, JCLDateTime.Pas unit

Call MSecsToTimeStamp to convert MSecs, a number of milliseconds, into the number of days represented, plus the number of milliseconds remaining. The days and milliseconds are expressed as a TTimeStamp value, where the Date field is the number of days and the Time field is the remainder in milliseconds.   (Unclear why Comp type is used in D4 and after when Int64 is available.)

VAR
  Frequency:  Int64;
  Start    :  Int64;    // D4-D5
  Stop     :  Int64;
...
QueryPerformanceFrequency(Frequency);
QueryPerformanceCounter(Start);
Sleep(2000);   // 2000 milliseconds = 2 seconds
QueryPerformanceCounter(Stop);
ShowMessage( IntToStr(Stop-Start) + ' ' +
             IntToStr(Frequency) +  ' ' +
             Format( '%.6f', [ (Stop-Start)/Frequency ])  + 
             ' seconds')

Sample output:  2389176 1193180 2.002360 seconds

JCLCounter.pas unit is part of Delphi Jedi Code Library

See also GetTickCount and GetCycleCount

ReplaceDate changes the value of the DateTime parameter so that it has the same date as the NewDate parameter. The time portion of DateTime remains the same. ReplaceDate makes any necessary adjustments for negative dates (before 1900).

ReplaceTime changes the value of the DateTime parameter so that it has the same time as the NewTime parameter. The date portion of DateTime remains the same. ReplaceTime makes any necessary adjustments for negative dates. (before 1900)

Peter Below's UseNet Post about setting CreatedDateTime of a file

StrToDate converts a string to a date format.

In Delphi 4, the TwoDigitCenturyWindow global variable affects what StrToDate returns.  Here are some examples:

Jamie Frater's UseNet Post with UnixTimeToDateTime by code by Philippe Ranger showing use of SystemTimeToTzSpecifiedLocalTime

Delphi 5 Developer's Guide, pp. 291-292

Jamie Frater's UseNet Post with UnixTimeToDateTime by code by Philippe Ranger showing use of SystemTimeToTzSpecifiedLocalTime

TDateTime is just a floating-point Double! The integer part of the value, INT(x), is the number of days. The fractional part, FRAC(x), is a fractional part of a day. 86400*FRAC(x) yields the number of seconds.

In Delphi 1, Delphi defined a TDateTime as the number of days that have passed since 1/1/0001.  The fractional part represents a fractional part of a day.

In Delphi 2 and later, the TDateTime represents the number of days since 30 December 1899.  A TDateTime of 0 corresponds to 30 Dec 1899; a value of 1 is 31 Dec 1899 = 0 Jan 1900.

A TDateTime can be any date through 31 Dec 9999 (decimal value 2,958,465).   You get the error "invalid argument to date encode" if you try EncodeDate(10000,1,1).

TDateTime values can also be negative.  The decimal value -693593, which can be represented as EncodeDate(1,1,1) or -DateDelta+1, corresponds to 1 Jan 0001. 

The DateDelta constant (693594) can be used to convert from D1 to post-D1 TDateTime values.

A Matter of Time:  Exploring the TDateTime Object, Delphi Informant, Nov 1996

See Borland's "TDateTime component commonly ignores key input," FAQ 966D

Displays a list box for entering dates or times.

Brian Bushay's UseNet Post about how to force the drop-down calendar to open.  Dave Nottage's UseNet Post about how to  force a TDateTimePicker to drop down programmatically.  See follow-up comments by Frank Aluttis with an improved way to do this.

In D6 and later, use the Format property to change display of date or time.  In D5 and earlier, use the DateTime_SetFormat call, as shown next:

How to have 4-digit years or 24-hour clocks with a TDateTimePicker:

4-Digit Year 
USES CommCtrl;
. . .
// put this in the FormCreate:

 
// 4-digit year for 3 y's (Microsoft's idea, not mine) 
// or 4 y's.  Example:   3/13/2000
// Use arrow keys to make selections field-by-field 
// (or use drop-down calendar)
DateTime_SetFormat(DateTimePickerBirthDate.Handle,
                   pChar('M/d/yyy'));

// Example:  13 Mar 2000
DateTime_SetFormat(DateTimePickerBirthDate.Handle, 
                   pChar('d MMM yyyy'));
// Unclear how to format "Today" above to be 6 Mar 2000.
// Could display dates with DateMode of dmUpDown to 
// suppress calendar:

24-Hour Clock

 
// 24-hour clock:  Be sure to set Kind to dtkTime
DateTime_SetFormat(DateTimePickerTest.Handle, pChar('H:mm:ss'));

Instead of using the DateTime_SetFormat function, you can send a message to the control directly:
SendMessage(DateTimePicker1.Handle, DTM_SETFORMAT, 0,
     DWORD(pChar('H:mm:ss')))

Both Date and Clock in Single Control?

// This appears to work:
DateTime_SetFormat(DateTimePickerTest.Handle, 
                   pChar('d MMM yyyy   H:mm:ss'));
// but you can only edit the date or the time depending 
// on the Kind (dtkTime or dtkDate).

Must a DateTimePicker always show a date?
Alan G Lloyd's UseNet Post about changing the format string to do this.

How can I display a blank date?

D6 Solution:

implementation
...

USES
  CommCtrl; // To set format of TDateTimePicker

// DateTimePicker1.Format is set in IDE to '' (quote-quote)

procedure TForm1.DateTimePicker1Change(Sender: TObject);
begin
  DateTimePicker1.Format := 'M/d/yyy';
end;

procedure TForm1.ButtonClearClick(Sender: TObject);
begin
  DateTimePicker1.Format := ''''''; // ''
end;

procedure TForm1.ButtonStatusClick(Sender: TObject);
begin
  IF DateTimePicker1.Format = ''''''
  THEN ShowMessage('Date is blank')
  ELSE ShowMessage('Date format is ' +
       DateTimePicker1.Format)
end;

Similar D5 Solution for blank dates without using D6 Format property (also works in D3 and D4).

Date property isn't updated while the calendar is dropped down (D4 and later)
Mark Reichert's UseNet Post about  the OnChange handler for the TDateTimePicker control DT_SelDate

A Better TDateTimePicker
www.bhnet.com.br/~simonet/tipstricks/pickerex.htm

Vanyó Tamás' UseNet Post with a TwoDBDateTimePicker to work as a TDBDateTimePicker

Note about D3 bug with TabStop (by Rick Rogers).

The following routines are from the CommCtrl unit (not to be confused with ComCtrls).  Be sure to add a USES CommCtrl statement to use these functions.  These routines appear to be equivalents of C macros (e.g., DateTime_SetFormat is a C macro for the DTM_SETFORMAT message).

function DateTime_GetSystemTime(hdp: HWND; var pst: TSystemTime): DWORD;

function DateTime_SetSystemTime(hdp: HWND; gd: DWORD; const pst: TSystemTime): BOOL;

function DateTime_GetRange(hdp: HWND; rgst: PSystemTime): DWORD;

function DateTime_SetRange(hdp: HWND; gdtr: DWORD; rgst: PSystemTime): BOOL;

function DateTime_SetFormat(hdp: HWND; sz: PChar): BOOL;
Format Characters (from Microsoft)

Example: (also see the FormCreate in AgeCalculation.TXT)

USES CommCtrl;
. . .
// 4-digit year for 3 y's
DateTime_SetFormat(DateTimePickerBirthDate.Handle,
                   pChar('M/dd/yyy'));  

DateTime_SetFormat(DateTimePickerTest.Handle,
                   pChar('H:mm:ss'));   // 24-hour clock

function DateTime_SetMonthCalColor(hdp: HWND; iColor: DWORD; clr: TColorRef): TColorRef;

function DateTime_GetMonthCalColor(hdp: HWND; iColor: DWORD): TColorRef;

function DateTime_GetMonthCal(hdp: HWND): HWND;

procedure DateTime_SetMonthCalFont(hdp: HWND; hfont: HFONT; fRedraw: BOOL);

function DateTime_GetMonthCalFont(hdp: HWND): HFONT;

Microsoft SDK information about Date and Time Picker Controls.

Date Picker Control Does Not Allow 4-Digit Year Input After 2035
http://support.microsoft.com/support/kb/articles/q223/2/92.asp

Formatting a DateTimePicker Control
www.undu.com/Articles/980707a.html

Enhancing the TDateTimePicker
www.undu.com/Articles/980714b.html

Number of 100-nanosecond intervals since January 1, 1601.

Typecast a TFileTime to an Int64 (D4 or later) to perform arithmetic.

Steve Schafer's UseNet Post about TFileTime to TDateTime

See GetThreadTimes.

Delphi 5 Developer's Guide, pp. 291-292

Section: TFileTime.  see above (Peter Haas' converting function use a lossless direct converting, only a addition and a multiplication)

Time Travels:  Of Time Zones, Daylight Savings, and other Delights, Delphi Informant, Feb 2000

Time Zones.  Borland's "Get a list of valid time zones from Windows"
FAQ 2700D

Registry Keys:
HKEY_LOCAL_MACHINE\System\CurrentControlSet\ Control\TimeZoneInformation

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Time Zones

Converts a TTimeStamp value into an absolute number of milliseconds.  (Unclear why Comp type is used in D4 and D5 when Int64 is available.)

Delphi 5 Developer's Guide, pp. 291-292

See also GetTimeZoneInformation

TimeZoneInformation = 
  RECORD
    Bias: Longint;
    StandardName: array[0..31] of WCHAR;
    StandardDate: TSystemTime;
    StandardBias: Longint;
    DaylightName: array[0..31] of WCHAR;
    DaylightDate: TSystemTime;
    DaylightBias: Longint;
  END;

From SysUtils Source:
"Determines what century is added to two digit years when converting string dates to numeric dates. This value is subtracted from the current year before extracting the century.  This can be used to extend the lifetime of existing applications that are inextricably tied to 2 digit year data entry. The best solution to Year 2000 (Y2k) issues is not to accept 2 digit years at all - require 4 digit years in data entry to eliminate century ambiguities."

Bill Todd's (Team B) UseNet Post explaining usage

See StrToDate above.

From Usenet Post by Finn Tolderlund:
FUNCTION UnixDateTimeToDelphiDateTime(UnixDateTime: LongInt): TDateTime;
BEGIN
  Result := EncodeDate(1970, 1, 1) + 
            (UnixDateTime / 86400); {86400=No. of secs. per day}
END;

function UnixToDateTime(const AValue: Int64): TDateTime; 

Poul Dige's UseNet Post with WeekNumber function

function ISOWeekNumber(const DateTime: TDateTime; 
                       var YearOfWeekNumber: Integer): Integer;
Delphi Jedi Code Library, JCLDateTime.Pas unit

Finn Tolderlund's UseNet Post with DateToWeek function

Info over different definitions and two lists, that list the used definition sorted by country resp. grouped by definition
http://www.pjh2.de/datetime/weeknumber/index.php

Week number unit, which can calculate a week number by definition,
inclusive the UK Inland Revenue Tax Weeks
Download: http://delphi.pjh2.de/units/download/DTWeeks.zip

Getting the date range of a given week number
http://members.xoom.com/_XMCM/jescott/DelphiCodeTips2.html#GetDates

USES
  SysUtils;  // DayOfWeek, DecodeDate, EncodeDate

// Returns Week Number for a given TDateTime.
// Assumes a Week Starts on Monday.
FUNCTION WeekNumber(TargetDate: TDateTime): INTEGER;
  CONST
    MondayIndex = 2;  // Sunday = 1
  VAR
    Year       :  WORD;
    Month      :  WORD;
    Day        :  WORD;
    MondayWeek1:  TDateTime;
BEGIN
  DecodeDate(TargetDate, Year, Month, Day);
  MondayWeek1 := EncodeDate(Year, 1, 1);
  WHILE DayOfWeek(MondayWeek1) <> MondayIndex DO
    MondayWeek1 := MondayWeek1 - 1;
  RESULT := ((TRUNC(TargetDate - MondayWeek1) DIV 7) MOD 52) + 1
END;

Source:
Frederic Guillen, E-Mail: fguillien@hol.fr
Used with permission. (Rewritten to use English mnemonics)

function YearOfDate(const DateTime: TDateTime): Integer; 

Delphi Jedi Code Library, JCLDateTime.Pas unit

Lars Ladingkaer's UseNet Post

UDDF, Section 12, Item 1
Unofficial Delphi Developers FAQ: www.gnomehome.demon.nl/uddf

Easter for years 1875 to 2124

ShowMessage( FormatDateTime('dd mmm yyyy, hh:nn:ss', newdt) );

Oliver Montenbruck and Thomas Pfleger, Astronomy on the Personal Computer (3rd edition), Springer-Verlag, 1998.  (Includes disk with Turbo Pascal 6 code)

Jean Meeus, Astronomical Algorithms, Willmann-Bell, Inc., 1991.
Chapter   7:  Julian Day
Chapter   9:  Dynamical Time and Universal Time
Chapter 11:  Sideral Time at Greenwich
Chapter 27:  Equation of Time (including equinoxes and solstices)

The Astronmical Almanac for the Year 2000 published by U.S. Naval Observatory/U.K. Royal Greenwich Observatory

Calendars and calendar-related utilities
http://garbo.uwasa.fi/pc/calendar.html

Calendar component suite.  Specializing in the quick selection of a period - left click the start date and right click the end   date, or drag a range.  http://delphi.icm.edu.pl/ftp/d10free/odcal25.zip

International Standard ISO 8601
ftp://ftp.qsl.net/pub/g1smd/8601.pdf

E-mail from Ian Galpin with ISO 8601 Info and links

The second edition of ISO 8601:
ftp://ftp.qsl.net/pub/g1smd/8601v03.pdf

A Summary of the International Standard Date and Time Notation
www.cl.cam.ac.uk/~mgk25/iso-time.html 

Date and Time Formats
www.w3.org/TR/NOTE-datetime

Datum und Zeit - ISO 8601 by Peter Haas
http://home.t-online.de/home/PeterJHaas/ISO8601.htm)

Campaign to get the Internet World to use the International Date Format ISO 8601
www.saqqara.demon.co.uk/datefmt.htm 

International Currency, Date, and Time Formats
http://msdn.microsoft.com/library/books/devintl/s25c7.htm 

TMoon component shows phases of the moon
www.hoerstemeier.com/delphi.htm#moon