mo_datetime: better conversion from string

• better conversion from string
• allow 'T' as time separator
• allow missing entries (e.g. no second given)

src/pf_tests/test_mo_datetime.pf

@@ -252,7 +252,8 @@ contains
     @assertTrue(datetime("1992-10-08 15:15:00") == datetime("1992-10-08 15:15"))
     @assertTrue(datetime("1992-10-08 15:00:00") == datetime("1992-10-08 15"))
     @assertTrue(datetime("1992-10-08T00:00:00") == datetime("1992-10-08"))
-    @assertTrue(datetime("1992-10-08T00:00:00") == datetime("1992-10-08"))
+    @assertTrue(datetime("1992-10-01T00:00:00") == datetime("1992-10"))
+    @assertTrue(datetime("1992-01-01T00:00:00") == datetime("1992"))
     @assertTrue(datetime("1992-10-01 15:15:00") == datetime("1992-10T15:15"))
     @assertTrue(datetime("1992-01-01 15:15:00") == datetime("1992 15:15"))