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

Instead, use IncHour with a negative increment.
 
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 := EncodeDateTime(2000, 12, 31, 23, 0, 0, 0);
  WriteLn('myDate = '+DateTimeToStr(myDate));

  // Add 10 hours to this date
  myDate := IncHour(myDate, 10);
  WriteLn('myDate + 10 hours = '+DateTimeToStr(myDate));

  // Subtract 10 hours from this date
  myDate := IncHour(myDate, -10);
  WriteLn('myDate - 10 hours = '+DateTimeToStr(myDate));
end;
Show full unit code
   myDate = 31/12/2000 23:00:00
   myDate + 10 hours = 01/01/2001 10:00:00
   myDate - 10 hours = 01/01/2001 13:00:00