Here's the function, its two helpers, and all the code, I wrote to parse a string using an exact datetime format:
class function TDateTimeUtils.TryStrToDateExact(const S, DateFormat: string; PivotYear: Integer;
out Value: TDateTime): Boolean;
var
Month, Day, Year: Integer;
Tokens: TStringDynArray;
CurrentToken: string;
i, n: Integer;
Partial: string;
MaxValue: Integer;
nCurrentYear: Integer;
function GetCurrentYear: Word;
var
y, m, d: Word;
begin
DecodeDate(Now, y, m, d);
Result := y;
end;
begin
Result := False;
{
M/dd/yy
Valid pictures codes are
d Day of the month as digits without leading zeros for single-digit days.
dd Day of the month as digits with leading zeros for single-digit days.
ddd Abbreviated day of the week as specified by a LOCALE_SABBREVDAYNAME* value, for example, "Mon" in English (United States).
Windows Vista and later: If a short version of the day of the week is required, your application should use the LOCALE_SSHORTESTDAYNAME* constants.
dddd Day of the week as specified by a LOCALE_SDAYNAME* value.
M Month as digits without leading zeros for single-digit months.
MM Month as digits with leading zeros for single-digit months.
MMM Abbreviated month as specified by a LOCALE_SABBREVMONTHNAME* value, for example, "Nov" in English (United States).
MMMM Month as specified by a LOCALE_SMONTHNAME* value, for example, "November" for English (United States), and "Noviembre" for Spanish (Spain).
y Year represented only by the last digit.
yy Year represented only by the last two digits. A leading zero is added for single-digit years.
yyyy Year represented by a full four or five digits, depending on the calendar used. Thai Buddhist and Korean calendars have five-digit years. The "yyyy" pattern shows five digits for these two calendars, and four digits for all other supported calendars. Calendars that have single-digit or two-digit years, such as for the Japanese Emperor era, are represented differently. A single-digit year is represented with a leading zero, for example, "03". A two-digit year is represented with two digits, for example, "13". No additional leading zeros are displayed.
yyyyy Behaves identically to "yyyy".
g, gg Period/era string formatted as specified by the CAL_SERASTRING value.
The "g" and "gg" format pictures in a date string are ignored if there is no associated era or period string.
PivotYear
The maximum year that a 1 or 2 digit year is assumed to be.
The Microsoft de-factor standard for y2k is 2029. Any value greater
than 29 is assumed to be 1930 or higher.
e.g. 2029:
1930, ..., 2000, 2001,..., 2029
If the PivotYear is between 0 and 99, then PivotYear is assumed to be
a date range in the future. e.g. (assuming this is currently 2010):
Pivot Range
0 1911..2010 (no future years)
1 1912..2011
...
98 2009..2108
99 2010..2099 (no past years)
0 ==> no years in the future
99 ==> no years in the past
}
if Length(S) = 0 then
Exit;
if Length(DateFormat) = 0 then
Exit;
Month := -1;
Day := -1;
Year := -1;
Tokens := TDateTimeUtils.TokenizeFormat(DateFormat);
n := 1; //input string index
for i := Low(Tokens) to High(Tokens) do
begin
CurrentToken := Tokens[i];
if CurrentToken = 'MMMM' then
begin
//Long month names, we don't support yet (you're free to write it)
Exit;
end
else if CurrentToken = 'MMM' then
begin
//Short month names, we don't support yet (you're free to write it)
Exit;
end
else if CurrentToken = 'MM' then
begin
//Month, with leading zero if needed
if not ReadDigitString(S, n, 2{MinDigits}, 2{MaxDigits}, 1{MinValue}, 12{MaxValue}, {var}Month) then Exit;
end
else if CurrentToken = 'M' then
begin
//months
if not ReadDigitString(S, n, 1{MinDigits}, 2{MaxDigits}, 1{MinValue}, 12{MaxValue}, {var}Month) then Exit;
end
else if CurrentToken = 'dddd' then
begin
Exit; //Long day names, we don't support yet (you're free to write it)
end
else if CurrentToken = 'ddd' then
begin
Exit; //Short day names, we don't support yet (you're free to write it);
end
else if CurrentToken = 'dd' then
begin
//If we know what month it is, and even better if we know what year it is, limit the number of valid days to that
if (Month >= 1) and (Month <= 12) then
begin
if Year > 0 then
MaxValue := MonthDays[IsLeapYear(Year), Month]
else
MaxValue := MonthDays[True, Month]; //we don't know the year, assume it's a leap year to be more generous
end
else
MaxValue := 31; //we don't know the month, so assume it's the largest
if not ReadDigitString(S, n, 2{MinDigits}, 2{MaxDigits}, 1{MinValue}, MaxValue{MaxValue}, {var}Day) then Exit;
end
else if CurrentToken = 'd' then
begin
//days
//If we know what month it is, and even better if we know what year it is, limit the number of valid days to that
if (Month >= 1) and (Month <= 12) then
begin
if Year > 0 then
MaxValue := MonthDays[IsLeapYear(Year), Month]
else
MaxValue := MonthDays[True, Month]; //we don't know the year, assume it's a leap year to be more generous
end
else
MaxValue := 31; //we don't know the month, so assume it's the largest
if not ReadDigitString(S, n, 1{MinDigits}, 2{MaxDigits}, 1{MinValue}, MaxValue{MaxValue}, {var}Day) then Exit;
end
else if (CurrentToken = 'yyyy') or (CurrentToken = 'yyyyy') then
begin
//Year represented by a full four or five digits, depending on the calendar used.
{
Thai Buddhist and Korean calendars have five-digit years.
The "yyyy" pattern shows five digits for these two calendars,
and four digits for all other supported calendars.
Calendars that have single-digit or two-digit years, such as for
the Japanese Emperor era, are represented differently.
A single-digit year is represented with a leading zero, for
example, "03". A two-digit year is represented with two digits,
for example, "13". No additional leading zeros are displayed.
}
if not ReadDigitString(S, n, 4{MinDigits}, 4{MaxDigits}, 0{MinValue}, 9999{MaxValue}, {var}Year) then Exit;
end
else if CurrentToken = 'yyy' then
begin
//i'm not sure what this would look like, so i'll ignore it
Exit;
end
else if CurrentToken = 'yy' then
begin
//Year represented only by the last two digits. A leading zero is added for single-digit years.
if not ReadDigitString(S, n, 2{MinDigits}, 2{MaxDigits}, 0{MinValue}, 99{MaxValue}, {var}Year) then Exit;
nCurrentYear := GetCurrentYear;
Year := (nCurrentYear div 100 * 100)+Year;
if (PivotYear < 100) and (PivotYear >= 0) then
begin
//assume pivotyear is a delta from this year, not an absolute value
PivotYear := nCurrentYear+PivotYear;
end;
//Check the pivot year value
if Year > PivotYear then
Year := Year - 100;
end
else if CurrentToken = 'y' then
begin
//Year represented only by the last digit.
if not ReadDigitString(S, n, 1{MinDigits}, 1{MaxDigits}, 0{MinValue}, 9{MaxValue}, {var}Year) then Exit;
nCurrentYear := GetCurrentYear;
Year := (nCurrentYear div 10 * 10)+Year;
if (PivotYear < 100) and (PivotYear >= 0) then
begin
//assume pivotyear is a delta from this year, not an absolute value
PivotYear := nCurrentYear+PivotYear;
end;
//Check the pivot year value
if Year > PivotYear then
Year := Year - 100;
end
else
begin
//The input string should contains CurrentToken starting at n
Partial := Copy(S, n, Length(CurrentToken));
Inc(n, Length(CurrentToken));
if Partial <> CurrentToken then
Exit;
end;
end;
//If there's still stuff left over in the string, then it's not valid
if n <> Length(s)+1 then
begin
Result := False;
Exit;
end;
if Day > MonthDays[IsLeapYear(Year), Month] then
begin
Result := False;
Exit;
end;
try
Value := EncodeDate(Year, Month, Day);
except
Result := False;
Exit;
end;
Result := True;
end;
class function TDateTimeUtils.TokenizeFormat(fmt: string): TStringDynArray;
var
i: Integer;
partial: string;
function IsDateFormatPicture(ch: AnsiChar): Boolean;
begin
case ch of
'M','d','y': Result := True;
else Result := False;
end;
end;
begin
SetLength(Result, 0);
if Length(fmt) = 0 then
Exit;
//format is only one character long? If so then that's the tokenized entry
if Length(fmt)=1 then
begin
SetLength(Result, 1);
Result[0] := fmt;
end;
partial := fmt[1];
i := 2;
while i <= Length(fmt) do
begin
//If the characters in partial are a format picture, and the character in fmt is not the same picture code then write partial to result, and reset partial
if IsDateFormatPicture(partial[1]) then
begin
//if the current fmt character is different than the running partial picture
if (partial[1] <> fmt[i]) then
begin
//Move the current partial to the output
//and start a new partial
SetLength(Result, Length(Result)+1);
Result[High(Result)] := partial;
Partial := fmt[i];
end
else
begin
//the current fmt character is more of the same format picture in partial
//Add it to the partial
Partial := Partial + fmt[i];
end;
end
else
begin
//The running partial is not a format picture.
//If the current fmt character is a picture code, then write out the partial and start a new partial
if IsDateFormatPicture(fmt[i]) then
begin
//Move the current partial to the output
//and start a new partial
SetLength(Result, Length(Result)+1);
Result[High(Result)] := partial;
Partial := fmt[i];
end
else
begin
//The current fmt character is another non-picture code. Add it to the running partial
Partial := Partial + fmt[i];
end;
end;
Inc(i);
Continue;
end;
//If we have a running partial, then add it to the output
if partial <> '' then
begin
SetLength(Result, Length(Result)+1);
Result[High(Result)] := partial;
end;
end;
class function TDateTimeUtils.ReadDigitString(const S: string; var Pos: Integer;
MinDigits, MaxDigits: Integer; MinValue, MaxValue: Integer;
var Number: Integer): Boolean;
var
Digits: Integer;
Value: Integer;
Partial: string;
CandidateNumber: Integer;
CandidateDigits: Integer;
begin
Result := False;
CandidateNumber := -1;
CandidateDigits := 0;
Digits := MinDigits;
while Digits <= MaxDigits do
begin
Partial := Copy(S, Pos, Digits);
if Length(Partial) < Digits then
begin
//we couldn't get all we wanted. We're done; use whatever we've gotten already
Break;
end;
//Check that it's still a number
if not TryStrToInt(Partial, Value) then
Break;
//Check that it's not too big - meaning that getting anymore wouldn't work
if (Value > MaxValue) then
Break;
if (Value >= MinValue) then
begin
//Hmm, looks good. Keep it as our best possibility
CandidateNumber := Value;
CandidateDigits := Digits;
end;
Inc(Digits); //try to be greedy, grabbing even *MORE* digits
end;
if (CandidateNumber >= 0) or (CandidateDigits > 0) then
begin
Inc(Pos, CandidateDigits);
Number := CandidateNumber;
Result := True;
end;
end;
