ObjectPascal
IncDay
Function
Increments a TDateTime variable by + or - number of days DateUtils unit
 function IncDay ( const StartDateTime : TDateTime {; NumberOfDays : Integer = 1} ) : TDateTime;
Description
The IncDay function returns a TDateTime value that is NumberOfDays greater than the passed StartDateTime value.
 
The year and month values are incremented as appropriate.
 
The increment value is optional, being 1 by default.
Notes
There is no DecDay function.

Instead, use IncDay with a negative increment.
Related commands
IncMinute Increments a TDateTime variable by + or - number of minutes
IncMonth Increments a TDateTime variable by a number of months
IncYear Increments a TDateTime variable by a number of years
IncSecond Increments a TDateTime variable by + or - number of seconds
IncMillisecond Increments a TDateTime variable by + or - number of milliseconds
 
Example code : A simple example of increment and decrement
var
  myDate : TDateTime;
begin
  // Set up our date just before the end of the year 2000
  myDate := EncodeDate(2000, 12, 30);
  WriteLn('myDate = '+DateToStr(myDate));

  // Add 10 days to this date
  myDate := IncDay(myDate, 10);
  WriteLn('myDate + 10 days = '+DateToStr(myDate));

  // Subtract 12 days from this date
  myDate := IncDay(myDate, -12);
  WriteLn('myDate - 12 days = '+DateToStr(myDate));
end;
Show full unit code
   myDate = 30/12/2000
   myDate + 10 days = 09/01/2001
   myDate - 12 days = 29/12/2000