Date-Time

Date-Time

The TDateTime data typeDate and time processing depends on the TDateTime variable. It is used to hold a date and time combination. It is also used to hold just date or time values - the time and date value is ignored respectively. TDateTime is defined in the System unit. Date constants and routines are defined in SysUtils and DateUtils units.
Let us look at some simple examples of assigning a value to a TDateTime variable:

var date1, date2, date3 : TDateTime; // TDateTime variables begin date1 := Yesterday;// Set to the start of yesterday date2 := Date; // Set to the start of the current day date3 := Tomorrow; // Set to the start of tomorrow date4 := Now;// Set to the current day and time end;

date1 is set to something like 12/12/2002 00:00:00 date2 is set to something like 13/12/2002 00:00:00 date3 is set to something like 14/12/2002 00:00:00 date4 is set to something like 13/12/2002 08:15:45

Note : the start of the day is often called midnight in Delphi documentation, but this is misleading, since it would be midnight of the wrong day.

Some named date valuesDelphi provides some useful day and month names, saving you the tedium of defining them in your own code. Here they are:
Short and long month names Note that these month name arrays start with index = 1.

var month : Integer; begin for month := 1 to 12 do // Display the short and long month names begin ShowMessage ShortMonthNames[month]); ShowMessage LongMonthNames[month]); end; end;

The ShowMessage routine display the following information: Jan January Feb February Mar March Apr April May May Jun June Jul July Aug August Sep September Oct October Nov November Dec December

Short and long day namesIt is important to note that these day arrays start with index 1 = Sunday. This is not a good standard (it is not ISO 8601 compliant), so be careful when using with ISO 8601 compliant routines such as DayOfTheWeek

var day : Integer; begin for day := 1 to 12 do // Display the short and long day names begin ShowMessage(ShortDayNames[day]); ShowMessage(LongDayNames[day]); end; end;

The ShowMessage routine display the following information: Sun Sunday Mon Monday Tue Tuesday Wed Wednesday Thu Thursday Fri Friday Sat Saturday

Date and time calculationsThe largest benefit of TDateTime is the range of calculations Delphi can do for you. These can be found on the Delphi Basics home page, in the Dates and Times/Calculations option.
In the following examples, click on the name to learn more:

DayOfTheMonthGives the day of month index for a TDateTime value DaysBetweenGives the whole number of days between 2 dates DaysInAMonth Gives the number of days in a month DaysInAYearGives the number of days in a year DecodeDate Extracts the year, month, day values from a TDateTime var. EncodeDate Build a TDateTime value from year, month and day values IncDay Increments a TDateTime variable by + or - number of days IsLeapYear Returns true if a given calendar year is a leap year MinsPerDay Gives the number of minutes in a day

Displaying date and time valuesThere are a number of routines that convert date and or time values to strings for display or file storage purposes, such as dateTimeToStr and TimeToString. But the most important is the FormatDateTime. It provides comprehensive formatting control, as illustrated by the following examples.

var myDate : TDateTime; begin // Set up our TDateTime variable with a full date and time : // 09/02/2000 at 05:06:07.008 (.008 milli-seconds) myDate := EncodeDateTime(2000, 2, 9, 5, 6, 7, 8); // Date only - numeric values with no leading zeroes (except year) ShowMessage('d/m/y = '+ FormatDateTime('d/m/y', myDate)); // Date only - numeric values with leading zeroes ShowMessage(' dd/mm/yy = '+ FormatDateTime('dd/mm/yy', myDate)); // Use short names for the day, month, and add freeform text ('of') ShowMessage('ddd d of mmm yyyy = '+ ); // Use long names for the day and month ShowMessage('dddd d of mmmm yyyy = ); // Use the ShortDateFormat settings only ShowMessage('ddddd = '+ FormatDateTime('ddddd', myDate)); // Use the LongDateFormat settings only ShowMessage(' dddddd = ' ); ShowMessage(''); // Time only - numeric values with no leading zeroes ); // Time only - numeric values with leading zeroes ShowMessage(' hh:nn:ss.zzz =); // Use the ShortTimeFormat settings only ShowMessage('t = ); // Use the LongTimeFormat settings only ShowMessage(' tt = ') // Use the ShortDateFormat + LongTimeFormat settings ShowMessage('c = '+ end;

The ShowMessage routine shows the following outputs : d/m/y = 9/2/00 dd/mm/yy = 09/02/00 ddd d of mmm yyyy = Wed 9 of Feb 2000 dddd d of mmmm yyyy = Wednesday 9 of February 2000 ddddd = 09/02/2000 dddddd = 09 February 2000 c = 09/02/2000 01:02:03 h:n:s.z = 1:2:3.4 hh:nn:ss.zzz = 01:02:03.004 t = 01:02 tt = 01:02:03 c = 09/02/2000 01:02:03

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