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TDateTime is stored as a double, the integer part representing days and the fractional part being fraction of a day. The integer part is the number of days that have passed since December 30, 1899, and can be a negative number. The fractional part reflects the fraction of a 24-hour day without regard to the sign of the TDateTime value, so care must be taken when computing negative TDateTime values with a fractional part.
Methods exist in Sysutils (datetimeroutines) that allow reading, writing and calculating with TDateTime, converting to and from a number of formats.
The following table displays examples of TDateTime values and their corresponding dates and times:

Double Value 'dd-mm-yyyy hh:nn:ss'
-1.75 29-12-1899 18:00:00
-1.5 29-12-1899 12:00:00
-1.25 29-12-1899 06:00:00
0 30-12-1899 00:00:00
1.25 31-12-1899 06:00:00
2.25 01-01-1900 06:00:00
25569 01-01-1970 00:00:00 (Unix Epoch)
36152.5 23-12-1998 12:00:00 (FPC 1.0 Beta2)
36526 01-01-2000 00:00:00
44927 01-01-2023 00:00:00

Get current time

Unit SysUtils function Now retrieves the current system date and time.

 function Now : TDateTime;

Adding and subtracting TDateTime

Unit DateUtils function DaysBetween tell number of whole days between two DateTime values.

  function DaysBetween ( const ToDate, FromDate : TDateTime ) : Integer;

Unit DateUtils function WeeksBetween tell number of whole weeks between two DateTime values.

  function WeeksBetween( const ToDate, FromDate : TDateTime ):Integer;

program DateProject1;

  DateFormatChars = 'dd"/"mm"/"yyyy';
  DateTime1, DateTime2: TDateTime;
  DateTime1 := now;
  DateTime2 := DateTime1 + 4*7;       //28 days later
  WriteLn('Current date is '+ FormatDateTime( DateFormatChars, DateTime1 ));
  WriteLn('28 days later date is '+ FormatDateTime( DateFormatChars, DateTime2 ));
  WriteLn('Number of days is '+ ( DaysBetween( DateTime1 ,DateTime2)).ToString );
  WriteLn('Number of weeks is '+ ( WeeksBetween( DateTime1 ,DateTime2)).ToString );

Set Date

Unit SysUtils function EncodeDate to set the date.

  function EncodeDate ( const Year, Month, Day : Word ) : TDateTime;

Compare two TDateTime

program CompareTwoDateTime;

  SysUtils, DateUtils;
   firstDate, secondDate: TDateTime;
  firstDate := EncodeDate(2000, 2, 29);
  secondDate := EncodeDate(2018, 8, 11);
  if DaysBetween( firstDate, secondDate) = 0
    then WriteLn('Both dates are same')
      if firstDate < secondDate
        then WriteLn('First date is earlier')
        else WriteLn('First date is later');

See also