Author: Hans Gulö

How to add a just a part of date/time (eg day, minute, or month) to a TDateTime 
type variable.

Answer:

I found VBScript's buildin function: DateAdd() is very handy. It allows you to 
specify which part-of-date you wish to add. 

Here's the Object Pascal version. I changed the name to DateTimeAdd() to make it 
more descriptive -- emphasizing that it works for DateTime instead of just Date. 
The original function expects a plain char type argument to specify the date part. 
I replaced that one with an enumeration type, ensuring the passed argument is in 
correct form during compile time. 

I'm not going to describe VBScript's DateAdd() further. Your knowledge about that 
function will help a bit, but know nothing about it is completely fine. 

1   uses
2     ..., SysUtils;
3   
4   type
5     TDateTimePart = (dtpHour, dtpMinute, dtpSecond, dtpMS, dtpDay, dtpMonth,
6       dtpYear);
7   
8   function DateTimeAdd(SrcDate: TDateTime; DatePart: TDateTimePart;
9     DiffValue: Integer): TDateTime;
10  
11  implementation
12  
13  function DateTimeAdd(SrcDate: TDateTime; DatePart: TDateTimePart;
14    DiffValue: Integer): TDateTime;
15  var
16    m, d, y: Word;
17  begin
18    case DatePart of
19      dtpHour: { hour }
20        Result := SrcDate + (DiffValue / 24);
21      dtpMinute: { Minute }
22        Result := SrcDate + (DiffValue / 1440);
23      dtpSecond: { Second }
24        Result := SrcDate + (DiffValue / 86400);
25      dtpMS: { Millisecond }
26        Result := SrcDate + (DiffValue / 86400000);
27      dtpDay: { Day }
28        Result := SrcDate + DiffValue;
29      dtpMonth: { Month }
30        Result := IncMonth(SrcDate, DiffValue);
31    else { Year }
32      begin
33        DecodeDate(SrcDate, y, m, d);
34        Result := Trunc(EncodeDate(y + DiffValue, m, d)) +
35          Frac(SrcDate);
36      end;
37    end; {case}
38  end;
39  
40  Sample: 
41  
42  var
43    Date3MonthsAfterNow: TDateTime;
44    Date2YearsAgo: TDateTime;
45    Date11DaysAfterNow: TDateTime;
46  begin
47    Date3MonthsAfterNow := DateTimeAdd(Now, dtpMonth, 3);
48    Date2YearsAgo := DateTimeAdd(Now, dtpYear, -2); // negative is OK
49    Date11DaysAfterNow := DateTimeAdd(Now, dtpDay, 11);
50  end;