DateTime String (any format) to TDateTime

Problem/Question/Abstract:

When extracting data from text or other operating systems the format of date strings can vary dramatically. Borland function StrToDateTime() converts a string to a TDateTime value, but it is limited to the fact that the string parameter must be in the format of the current locale’s date/time format.

eg. "MM/DD/YY HH:MM:SS"

Answer:
This is of little use when extracting dates such as ..

        1) "Friday 18 October 2002 08:34am (45 secs)"  or "Wednesday 15 May 2002 06:12 (22 secs)"
        2) "20020431"
        3) "12.Nov.03"
        4) "14 Hour 31 Minute 25 Second 321 MSecs"

This function will evaluate a DateTime string in accordance to the DateTime specifier format string supplied. The following specifiers are supported ...

  dd                                 the day as a number with a leading zero or space (01-31).  
  ddd                         the day as an abbreviation (Sun-Sat)
  dddd                         the day as a full name (Sunday-Saturday)
  mm                         the month as a number with a leading zero or space (01-12).
  mmm                 the month as an abbreviation (Jan-Dec)
  mmmm                 the month as a full name (January-December)
  yy                                 the year as a two-digit number (00-99).
  yyyy                         the year as a four-digit number (0000-9999).
  hh                                 the hour with a leading zero or space (00-23)
  nn                                 the minute with a leading zero or space (00-59).
  ss                                 the second with a leading zero or space (00-59).
  zzz                                 the millisecond with a leading zero (000-999).
  ampm                 Specifies am or pm flag hours (0..12)
  ap                                 Specifies a or p flag hours (0..12)
  (Any other character corresponds to a literal or delimiter.)

NOTE : One assumption I have to make is that DAYS, MONTHS, HOURS and MINUTES have a leading                       ZERO or SPACE (ie. are 2 chars long) and MILLISECONDS are 3 chars long (ZERO or SPACE                        padded)

Using function
DateTimeStrEval(const DateTimeFormat : string; const DateTimeStr : string) : TDateTime;

The above Examples (1..4) can be evaluated as ... (Assume DT1 to DT4 equals example strings 1..4)

        1)MyDate := DateTimeStrEval('dddd dd mmmm yyyy hh:nnampm (ss xxxx)', DT1);
        2)MyDate := DateTimeStrEval('yyyymmdd', DT2);
        3)MyDate := DateTimeStrEval('dd-mmm-yy', DT3);
        4)MyDate := DateTimeStrEval('hh xxxx nn xxxxxx ss xxxxxx zzz xxxxx', DT4);

uses SysUtils, DateUtils

  // =============================================================================
  // Evaluate a date time string into a TDateTime obeying the
  // rules of the specified DateTimeFormat string
  // eg. DateTimeStrEval('dd-MMM-yyyy hh:nn','23-May-2002 12:34)
  //
  // Delphi 6 Specific in DateUtils can be translated to ....
  //
  // YearOf()
  //
  // function YearOf(const AValue: TDateTime): Word;
  // var LMonth, LDay : word;
  // begin
  //   DecodeDate(AValue,Result,LMonth,LDay);
  // end;
  //
  // TryEncodeDateTime()
  //
  // function TryEncodeDateTime(const AYear,AMonth,ADay,AHour,AMinute,ASecond,
  //                            AMilliSecond : word;
  //                            out AValue : TDateTime): Boolean;
  // var LTime : TDateTime;
  // begin
  //   Result := TryEncodeDate(AYear, AMonth, ADay, AValue);
  //   if Result then begin
  //     Result := TryEncodeTime(AHour, AMinute, ASecond, AMilliSecond, LTime);
  //     if Result then
  //        AValue := AValue + LTime;
  //   end;
  // end;
  //
  // (TryEncodeDate() and TryEncodeTime() is the same as EncodeDate() and
  //  EncodeTime() with error checking and boolean return value)
  //
  // =============================================================================

function DateTimeStrEval(const DateTimeFormat: string;
  const DateTimeStr: string): TDateTime;
var
  i, ii, iii: integer;
  Retvar: TDateTime;
  Tmp,
    Fmt, Data, Mask, Spec: string;
  Year, Month, Day, Hour,
    Minute, Second, MSec: word;
  AmPm: integer;
begin
  Year := 1;
  Month := 1;
  Day := 1;
  Hour := 0;
  Minute := 0;
  Second := 0;
  MSec := 0;
  Fmt := UpperCase(DateTimeFormat);
  Data := UpperCase(DateTimeStr);
  i := 1;
  Mask := '';
  AmPm := 0;

  while i < length(Fmt) do
  begin
    if Fmt[i] in ['A', 'P', 'D', 'M', 'Y', 'H', 'N', 'S', 'Z'] then
    begin
      // Start of a date specifier
      Mask := Fmt[i];
      ii := i + 1;

      // Keep going till not valid specifier
      while true do
      begin
        if ii > length(Fmt) then
          Break; // End of specifier string
        Spec := Mask + Fmt[ii];

        if (Spec = 'DD') or (Spec = 'DDD') or (Spec = 'DDDD') or
          (Spec = 'MM') or (Spec = 'MMM') or (Spec = 'MMMM') or
          (Spec = 'YY') or (Spec = 'YYY') or (Spec = 'YYYY') or
          (Spec = 'HH') or (Spec = 'NN') or (Spec = 'SS') or
          (Spec = 'ZZ') or (Spec = 'ZZZ') or
          (Spec = 'AP') or (Spec = 'AM') or (Spec = 'AMP') or
          (Spec = 'AMPM') then
        begin
          Mask := Spec;
          inc(ii);
        end
        else
        begin
          // End of or Invalid specifier
          Break;
        end;
      end;

      // Got a valid specifier ? - evaluate it from data string
      if (Mask <> '') and (length(Data) > 0) then
      begin
        // Day 1..31
        if (Mask = 'DD') then
        begin
          Day := StrToIntDef(trim(copy(Data, 1, 2)), 0);
          delete(Data, 1, 2);
        end;

        // Day Sun..Sat (Just remove from data string)
        if Mask = 'DDD' then
          delete(Data, 1, 3);

        // Day Sunday..Saturday (Just remove from data string LEN)
        if Mask = 'DDDD' then
        begin
          Tmp := copy(Data, 1, 3);
          for iii := 1 to 7 do
          begin
            if Tmp = Uppercase(copy(LongDayNames[iii], 1, 3)) then
            begin
              delete(Data, 1, length(LongDayNames[iii]));
              Break;
            end;
          end;
        end;

        // Month 1..12
        if (Mask = 'MM') then
        begin
          Month := StrToIntDef(trim(copy(Data, 1, 2)), 0);
          delete(Data, 1, 2);
        end;

        // Month Jan..Dec
        if Mask = 'MMM' then
        begin
          Tmp := copy(Data, 1, 3);
          for iii := 1 to 12 do
          begin
            if Tmp = Uppercase(copy(LongMonthNames[iii], 1, 3)) then
            begin
              Month := iii;
              delete(Data, 1, 3);
              Break;
            end;
          end;
        end;

        // Month January..December
        if Mask = 'MMMM' then
        begin
          Tmp := copy(Data, 1, 3);
          for iii := 1 to 12 do
          begin
            if Tmp = Uppercase(copy(LongMonthNames[iii], 1, 3)) then
            begin
              Month := iii;
              delete(Data, 1, length(LongMonthNames[iii]));
              Break;
            end;
          end;
        end;

        // Year 2 Digit
        if Mask = 'YY' then
        begin
          Year := StrToIntDef(copy(Data, 1, 2), 0);
          delete(Data, 1, 2);
          if Year < TwoDigitYearCenturyWindow then
            Year := (YearOf(Date) div 100) * 100 + Year
          else
            Year := (YearOf(Date) div 100 - 1) * 100 + Year;
        end;

        // Year 4 Digit
        if Mask = 'YYYY' then
        begin
          Year := StrToIntDef(copy(Data, 1, 4), 0);
          delete(Data, 1, 4);
        end;

        // Hours
        if Mask = 'HH' then
        begin
          Hour := StrToIntDef(trim(copy(Data, 1, 2)), 0);
          delete(Data, 1, 2);
        end;

        // Minutes
        if Mask = 'NN' then
        begin
          Minute := StrToIntDef(trim(copy(Data, 1, 2)), 0);
          delete(Data, 1, 2);
        end;

        // Seconds
        if Mask = 'SS' then
        begin
          Second := StrToIntDef(trim(copy(Data, 1, 2)), 0);
          delete(Data, 1, 2);
        end;

        // Milliseconds
        if (Mask = 'ZZ') or (Mask = 'ZZZ') then
        begin
          MSec := StrToIntDef(trim(copy(Data, 1, 3)), 0);
          delete(Data, 1, 3);
        end;

        // AmPm A or P flag
        if (Mask = 'AP') then
        begin
          if Data[1] = 'A' then
            AmPm := -1
          else
            AmPm := 1;
          delete(Data, 1, 1);
        end;

        // AmPm AM or PM flag
        if (Mask = 'AM') or (Mask = 'AMP') or (Mask = 'AMPM') then
        begin
          if copy(Data, 1, 2) = 'AM' then
            AmPm := -1
          else
            AmPm := 1;
          delete(Data, 1, 2);
        end;

        Mask := '';
        i := ii;
      end;
    end
    else
    begin
      // Remove delimiter from data string
      if length(Data) > 1 then
        delete(Data, 1, 1);
      inc(i);
    end;
  end;

  if AmPm = 1 then
    Hour := Hour + 12;
  if not TryEncodeDateTime(Year, Month, Day, Hour, Minute, Second, MSec, Retvar) then
    Retvar := 0.0;
  Result := Retvar;
end;