Date validation

[ranlaborde]

I have read the PDF regarding dates and have reviewed the functions and procedures in DateUtil but I still have a problem:

I expect users to use my program in several countries with different date orders.  The IsValidDate function requires knowing the Month day and year to pass to the function. As the order of these change according to the system settings of each computer I need an example of date validation that works across different international settings.

Could someone point me to an example?

[munair]

I use a procedure to isolate the date items stored in a string. I then validate the items based on the system's locale. Here is an idea of how you can do this:

Code: Pascal  [Select][+][-]

1.   // get date items
2.   s := Collect(source, tiNumberDigits);
3.   count := High(s) + 1;
4.   // define items
5.   if count = 2 then
6.     // only day and month
7.     begin
8.       // get current year
9.       d.Year := GetYear(SystemDate());
10.       if Notation.DateFormat = Notation_DMY then
11.         begin
12.           d.Day := IntVal(s[0]);
13.           d.Month := IntVal(s[1]);
14.         end
15.       else
16.         begin
17.           d.Month := IntVal(s[0]);
18.           d.Day := IntVal(s[1]);
19.         end;
20.     end
21.   else
22.     begin
23.       case Notation.DateFormat of
24.         Notation_DMY:
25.           begin
26.             d.Day := IntVal(s[0]);
27.             d.Month := IntVal(s[1]);
28.             d.Year := IntVal(s[2]);
29.           end;
30.         Notation_MDY:
31.           begin
32.             d.Day := IntVal(s[1]);
33.             d.Month := IntVal(s[0]);
34.             d.Year := IntVal(s[2]);
35.           end;
36.         Notation_YMD:
37.           begin
38.             d.Day := IntVal(s[2]);
39.             d.Month := IntVal(s[1]);
40.             d.Year := IntVal(s[0]);
41.           end;
42.       end;
43.     end;

The procedure Collect works as follows (you can write this yourself): go through each character of the string containing the date; as long as it is a number buffer it into an array element. If an alphanumeric characters is found, skip that character and increment the array index if the previous character was numeric. That will give you an array of three elements, provided the date is valid. This will give you maximum flexibility in treating the date elements. My example allows omitting the year, which will then be assumed to be the system's current year.

[wp]

I suppose you are talking about conversion of a date string to a TDate. The record FormatSettings contains a list of country-specific settings, among them the element ShortDateFormat which, in my country, is the string "dd.mm.yy". This means: two-digit-day, then two-digit-month, then two-digit year, separated by dots. The FormatSettings are initialized when the program starts according to the defaults of the operating system (in Linux/Unix the unit "clocale" must be added to "uses").

The function StrToDate converts a string to a date value based on the ShortDateFormat string. I.e, the date string of today, '21.07.17' will be converted correctly in my country. You could also call TryStrToDate which returns a boolean false if the string cannot be converted successfully, instead of raising an exception.

Both functions are overloaded with versions accepting a local copy of the format settings to try other formats. The following code, for example, would check the date separators '.'. '/', and '-':

Code: Pascal  [Select][+][-]

1. function MyStrToDate(s: String): TDate;
2. var
3.   fs: TFormatSettings;
4. begin
5.   // At first check default format
6.   if TryStrToDate(s, Result) then
7.     exit;
8.   fs := FormatSettings;
9.   // StrToDate replaces the slash ('/') by the DateSeparator of the FormatSettings.
10.   if pos('/', FormatSettings.ShortDateFormat) > 0 then begin
11.     if FormatSettings.DateSeparator <> '-' then begin
12.       fs.DateSeparator := '-';
13.       if TryStrToDate(s, result) then
14.         exit;
15.     end;
16.     if FormatSettings.DateSeparator <> '/' then begin
17.       fs.DateSeparator := '/';
18.       if TryStrToDate(s, Result) then
19.         exit;
20.     end;
21.     if FormatSettings.DateSeparator <> '.' then begin
22.       fs.Dateseparator := '.';
23.       if TryStrToDate(s, Result) then
24.         exit;
25.     end;
26.   end;
27.   // Now check cases where DateSeparator is hard-coded in ShortDateFormat
28.   if FormatSettings.DateSeparator <> '-' then begin
29.     fs.ShortDateFormat := StringReplace(FormatSettings.ShortDateFormat, FormatSettings.DateSeparator, '-', [rfReplaceAll]);
30.     if TryStrToDate(s, Result) then
31.       exit;
32.   end;
33.   if FormatSettings.DateSeparator <> '/' then begin
34.     fs.ShortDateFormat := StringReplace(FormatSettings.ShortDateFormat, FormatSettings.DateSeparator, '/',  [rfReplaceAll]);
35.     if TryStrToDate(s, Result) then
36.       exit;
37.   end;
38.   if FormatSettings.DateSeparator <> '.' then begin
39.     fs.ShortDateFormat := StringReplace(FormatSettings.ShortDateFormat, FormatSettings.DateSeparator, '.',  [rfReplaceAll]);
40.     if TryStrToDate(s, Result) then
41.       exit;
42.   end;
43.  
44.   // if we get here the string parts most probably do not lead to a valid date --> raise an exception
45.   raise EConvertError.Create('Invalid date string ' + s)
46. end;

 
The function (Try)StrToDate, and thus the function above, is flexible enough to accept also a 4-digit year, or a 1-digit day or month, e.g. '21.7.2017'.

Therefore, I think that above function will cover most of the cases.

[ranlaborde]

Thanks so much for the quick responses.  Artie, your solution is very much what I came up with myself.  What unit is Notation located in?  Looks quite useful.

wp:  I looked high and low for TryStrToDate.  This is very useful.  I appreciate the example of using FormatSettings as well as this addresses the problem of some users using Windows and others using Macs (not developing for Linux at this point).

Again, thanks for the responses!!

[munair]

It is not a unit that comes with Lazarus. I setup some notation definitions like:

Code: Pascal  [Select][+][-]

1. const
2.   Notation_MDY = 0;
3.   Notation_DMY = 1;
4.   Notation_YMD = 2;

and use that throughout my projects, including date and time formatting etc.