SQL: Fix parsing of dates with milliseconds (#30419)

Dates internally contain milliseconds (which appear when converting them
to Strings) however parsing does not accept them (and is being strict).
The parser has been changed so that Date is mandatory but the time
(including its fractions such as millis) are optional.

Fix #30002

x-pack/plugin/sql/src/main/java/org/elasticsearch/xpack/sql/type/DataTypeConversion.java

@@ -31,7 +31,7 @@
  */
 public abstract class DataTypeConversion {
 
-    private static final DateTimeFormatter UTC_DATE_FORMATTER = ISODateTimeFormat.dateTimeNoMillis().withZoneUTC();
+    private static final DateTimeFormatter UTC_DATE_FORMATTER = ISODateTimeFormat.dateOptionalTimeParser().withZoneUTC();
 
     /**
      * Returns the type compatible with both left and right types

x-pack/plugin/sql/src/test/java/org/elasticsearch/xpack/sql/type/DataTypeConversionTests.java

@@ -82,10 +82,15 @@ public void testConversionToDate() {
         Conversion conversion = DataTypeConversion.conversionFor(DataType.KEYWORD, to);
         assertNull(conversion.convert(null));
 
-        // TODO we'd like to be able to optionally parse millis here I think....
         assertEquals(new DateTime(1000L, DateTimeZone.UTC), conversion.convert("1970-01-01T00:00:01Z"));
         assertEquals(new DateTime(1483228800000L, DateTimeZone.UTC), conversion.convert("2017-01-01T00:00:00Z"));
         assertEquals(new DateTime(18000000L, DateTimeZone.UTC), conversion.convert("1970-01-01T00:00:00-05:00"));
+
+        // double check back and forth conversion
+        DateTime dt = DateTime.now(DateTimeZone.UTC);
+        Conversion forward = DataTypeConversion.conversionFor(DataType.DATE, DataType.KEYWORD);
+        Conversion back = DataTypeConversion.conversionFor(DataType.KEYWORD, DataType.DATE);
+        assertEquals(dt, back.convert(forward.convert(dt)));
         Exception e = expectThrows(SqlIllegalArgumentException.class, () -> conversion.convert("0xff"));
         assertEquals("cannot cast [0xff] to [Date]:Invalid format: \"0xff\" is malformed at \"xff\"", e.getMessage());
     }