Inverse function of FormatDateTime

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.

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');