ValidatePackageInfo accepts date format yyyy-mm-dd

If the date format is dd/mm/yyyy and InfoLevel InfoPackageLoading
is set to at least 2 it prints a hint to change the format.

Add validity check for dates

Update relevant testfile

Fixes gap-system#2727

lib/package.gi

@@ -2135,7 +2135,7 @@ InstallGlobalFunction( DeclareAutoreadableVariables,
 InstallGlobalFunction( ValidatePackageInfo, function( info )
     local record, pkgdir, i, IsStringList, IsRecordList, IsProperBool, IsURL,
           IsFilename, IsFilenameList, result, TestOption, TestMandat, subrec,
-          list;
+          list, CheckDateValidity;
 
     if IsString( info ) then
       if IsReadableFile( info ) then
@@ -2203,9 +2203,36 @@ InstallGlobalFunction( ValidatePackageInfo, function( info )
         x -> IsString(x) and 0 < Length(x) and x[1] <> '=',
         "a nonempty string that does not start with `='" );
     TestMandat( record, "Date",
-        x -> IsString(x) and Length(x) = 10 and x{ [3,6] } = "//"
-                 and ForAll( x{ [1,2,4,5,7,8,9,10] }, IsDigitChar ),
-        "a string of the form `dd/mm/yyyy'" );
+        x -> IsString(x) and Length(x) = 10
+          and ( ( x{ [3,6] } = "//" and ForAll( x{ [1,2,4,5,7,8,9,10] }, IsDigitChar ) )
+          or ( x{ [5,8] } = "--" and ForAll( x{ [1,2,3,4,6,7,9,10] }, IsDigitChar ) ) ) ,
+        "a string of the form `yyyy-mm-dd` or `dd/mm/yyyy`" );
+
+    #If the date is in the format `dd/mm/yyyy` a message is printed
+    if IsBound( record.Date ) and  record.Date{ [3,6] } = "//" then
+       Info( InfoPackageLoading, 2, Concatenation(record.PackageName, ": Please be adviced to change the date format to `yyyy-mm-dd`"));
+    fi;
+
+    #check if the date is valid
+    CheckDateValidity := function(x)
+      local date;
+      if x{ [3,6] } = "//" then #for the old format split at '/'
+        date := List( SplitString( x, "/" ), Int);
+      elif x{ [5,8] } = "--" then #for the yyyy-mm-dd format split at '-'
+        date := List( SplitString( date, "-" ), Int);
+        Permuted(date, (1,3)); #sort such that date=[dd,mm,yyyy]
+      fi;
+      if IsBound(date) and (not (date[1] in [1..31] and
+                   date[2] in [1..12] and
+                   date[3] > 1999 and # GAP 4 appeared in 1999
+                   date[1] <= DaysInMonth( date[2], date[3] ))) then
+              return false;
+      fi;
+    return true;
+    end;
+    TestMandat( record, "Date", 
+      x -> IsString(x) and CheckDateValidity(x), "a valid date");
+
     TestOption( record, "License",
         x -> IsString(x) and 0 < Length(x),
         "a nonempty string containing an SPDX ID" );

tst/testinstall/package.tst

@@ -202,7 +202,9 @@ gap> ValidatePackageInfo(rec());
 #E  component `Subtitle' must be bound to a string
 #E  component `Version' must be bound to a nonempty string that does not start\
  with `='
-#E  component `Date' must be bound to a string of the form `dd/mm/yyyy'
+#E  component `Date' must be bound to a string of the form `yyyy-mm-dd` or `dd\
+/mm/yyyy`
+#E  component `Date' must be bound to a valid date
 #E  component `ArchiveURL' must be bound to a string started with http://, htt\
 ps:// or ftp://
 #E  component `ArchiveFormats' must be bound to a string