Fix serialization

Author: luigidellaquila

server/src/main/java/org/elasticsearch/common/Rounding.java

@@ -172,7 +172,7 @@ long extraLocalOffsetLookup() {
                 return ratio;
             }
         },
-        YEARS_OF_CENTURY((byte) 9, "year", ChronoField.YEAR_OF_ERA, false, 12) {
+        YEARS_OF_CENTURY((byte) 9, "years", ChronoField.YEAR_OF_ERA, false, 12) {
             private final long extraLocalOffsetLookup = TimeUnit.DAYS.toMillis(366);
 
             @Override
@@ -185,7 +185,7 @@ long extraLocalOffsetLookup() {
                 return extraLocalOffsetLookup;
             }
         },
-        MONTHS_OF_YEAR((byte) 10, "month", ChronoField.MONTH_OF_YEAR, false, 1) {
+        MONTHS_OF_YEAR((byte) 10, "months", ChronoField.MONTH_OF_YEAR, false, 1) {
             private final long extraLocalOffsetLookup = TimeUnit.DAYS.toMillis(31);
 
             @Override
@@ -551,6 +551,12 @@ private LocalDateTime truncateLocalDateTime(LocalDateTime localDateTime) {
                 case YEAR_OF_CENTURY:
                     return LocalDateTime.of(LocalDate.of(localDateTime.getYear(), 1, 1), LocalTime.MIDNIGHT);
 
+                case YEARS_OF_CENTURY:
+                    return LocalDateTime.of(LocalDate.of(localDateTime.getYear(), 1, 1), LocalTime.MIDNIGHT);
+
+                case MONTHS_OF_YEAR:
+                    return LocalDateTime.of(localDateTime.getYear(), localDateTime.getMonthValue(), 1, 0, 0);
+
                 default:
                     throw new IllegalArgumentException("NOT YET IMPLEMENTED for unit " + unit);
             }
@@ -913,6 +919,8 @@ private LocalDateTime nextRelevantMidnight(LocalDateTime localMidnight) {
                     case MONTH_OF_YEAR -> localMidnight.plus(1, ChronoUnit.MONTHS);
                     case QUARTER_OF_YEAR -> localMidnight.plus(3, ChronoUnit.MONTHS);
                     case YEAR_OF_CENTURY -> localMidnight.plus(1, ChronoUnit.YEARS);
+                    case YEARS_OF_CENTURY -> localMidnight.plus(1, ChronoUnit.YEARS);
+                    case MONTHS_OF_YEAR -> localMidnight.plus(1, ChronoUnit.MONTHS);
                     default -> throw new IllegalArgumentException("Unknown round-to-midnight unit: " + unit);
                 };
             }

server/src/main/java/org/elasticsearch/search/aggregations/bucket/histogram/DateHistogramAggregationBuilder.java

@@ -70,7 +70,9 @@ public class DateHistogramAggregationBuilder extends ValuesSourceAggregationBuil
         entry("minute", Rounding.DateTimeUnit.MINUTE_OF_HOUR),
         entry("1m", Rounding.DateTimeUnit.MINUTE_OF_HOUR),
         entry("second", Rounding.DateTimeUnit.SECOND_OF_MINUTE),
-        entry("1s", Rounding.DateTimeUnit.SECOND_OF_MINUTE)
+        entry("1s", Rounding.DateTimeUnit.SECOND_OF_MINUTE),
+        entry("years", Rounding.DateTimeUnit.YEARS_OF_CENTURY),
+        entry("months", Rounding.DateTimeUnit.MONTHS_OF_YEAR)
     );
 
     public static final ObjectParser<DateHistogramAggregationBuilder, String> PARSER = ObjectParser.fromBuilder(