Inverse function of FormatDateTime

I am looking for a function to reverse any string 开发者_运维知识库(YYYYMDD,YY/MM/DD,YYMMDD,...) created by the function FormatDateTime to datetime.

example

I have a string-date in format YYYYMMDDcreated by FormatDateTime

mydatestr:=FormatDateTime('YYYYMMDD',Mydate); 

now how I can convert mydatestr to DateTime again?

UPDATE

these functions

function StrToDate(const S: string): TDateTime; overload;
function StrToDate(const S: string;
  const FormatSettings: TFormatSettings): TDateTime; overload;

function StrToDateTime(const S: string): TDateTime; overload;
function StrToDateTime(const S: string;
  const FormatSettings: TFormatSettings): TDateTime; overload;

do not support passing a string with the format to convert.

I am looking something like this

Mydatetime:=InvFormatDatetime('20091225','yyyymmdd');

or

Mydatetime:=InvFormatDatetime('20090108','yyyyddmm');

---

It is quite easy with existing solution, StrToDateFmt function in rxDateutil.pas unit from RX package, which can be downloaded here: http://sourceforge.net/projects/rxlib/

EDIT:

Mentioned above function and StrToDateFmt from rxDateutil.pas are doing exactly what you expect, converting string to datetime using specified string mask, the code is too large to be included as this unit contains also other date functions, some of them required for converting string to date.

Example of use:

Result := StrToDateFmtDef('MM/DD/YYYY', '11/11/2011', Now);

---

Note: Unfortunately, as pointed out by Martien in this answer's comments, this solution will not work for cases where the date includes no date separator. However, I'm going to leave the answer up for anyone that may find it useful regardless.

I liked Tuncay's answer but there were a couple of problems with it. I'd have left a comment but I don't have enough reputation points.

So, here's the corrected version of Tuncay's answer (amending "TFormatSetting" missing an "s" and specified the format settings date separator):

function AnyStringToDate(fmt, dt: String) : TDateTime;
var
  fs : TFormatSettings;
Begin
  fs := TFormatSettings.Create;
  fs.DateSeparator := '-';
  fs.ShortDateFormat := fmt;

  result := StrToDateDef(dt, 0, fs);
End;

Identical to Tuncay's answer, AnyStringToDate can be used as follows:

mydate := AnyStringToDate('YYYY-MM-DD', '2015-01-20');

---

Without using any external library, you can do something like:

function AnyStringToDate(fmt, dt: String) : TDateTime;
var
  fs : TFormatSettings;
Begin
  fs := TFormatSettings.Create;
  fs.ShortDateFormat := fmt;

  result := StrToDateDef(dt, 0, fs);
End;

and then use it like:

  mydate := AnyStringToDate('YYYY-MM-DD', '2015-01-20');

I havent compiled this, but the idea simple.

---

You could use StrToDateFmt function of JvJCLUtils unit belonging to JEDI Code Library

in your case :

Function InvFormatDatetime ( dateString :String; dateFormat :String ) : TDateTime;
begin
   Result := JvJCLUtils.StrToDateFmt ( dateFormat, dateString );
end;

---

Did you check StrToDate and StrToDateTime ?

---

I know its too late but just for interest sake, with Delphi XE6 onwards you can now do the following

uses
  System.DateUtils;
var
  dt: TDateTime;
begin
  dt := ISO8601ToDate('20190408');
end.

dt will now be 2019/04/08

---

Function InvFormatDatetime (Cadena:String; Formato:String) : TDateTime;

Var
  PosD, PosM, PosY : Integer;
  sD, sM, sY       : String;

begin

  sd := '0';
  sm := '0';
  sy := '0';

  If Length(Cadena) = Length(Formato) Then
    Begin
      Formato := UpperCase(Formato);
      PosD := Pos('D',Formato);
      PosM := Pos('M',Formato);
      PosY := Pos('Y',Formato);

      sd := Copy(Cadena,PosD,2);
      sm := Copy(Cadena,PosM,2);

      if Length(Cadena) = 6 then
        begin
          sy := Copy(Cadena,PosY,2);
          if StrToInt(sy) > 50 then
            sy := '19'+sy
          else
            sy := '20'+sy;
        end
      else
        sy := Copy(Cadena,Posy,4);
    End;
  Result := EncodeDate(StrToInt(sY),
                       StrToInt(sM),
                       StrToInt(sD));
End;

greetings