diff --git a/x-pack/plugins/stack_alerts/common/build_sorted_events_query.test.ts b/x-pack/plugins/stack_alerts/common/build_sorted_events_query.test.ts
index bac84bd4a4b28..8e53736b23963 100644
--- a/x-pack/plugins/stack_alerts/common/build_sorted_events_query.test.ts
+++ b/x-pack/plugins/stack_alerts/common/build_sorted_events_query.test.ts
@@ -63,6 +63,7 @@ describe('buildSortedEventsQuery', () => {
         sort: [
           {
             timefield: {
+              format: 'strict_date_optional_time||epoch_millis',
               order: 'asc',
             },
           },
@@ -111,6 +112,7 @@ describe('buildSortedEventsQuery', () => {
         sort: [
           {
             timefield: {
+              format: 'strict_date_optional_time||epoch_millis',
               order: 'asc',
             },
           },
@@ -160,6 +162,7 @@ describe('buildSortedEventsQuery', () => {
         sort: [
           {
             timefield: {
+              format: 'strict_date_optional_time||epoch_millis',
               order: 'asc',
             },
           },
@@ -210,6 +213,7 @@ describe('buildSortedEventsQuery', () => {
         sort: [
           {
             timefield: {
+              format: 'strict_date_optional_time||epoch_millis',
               order: 'asc',
             },
           },
@@ -272,6 +276,7 @@ describe('buildSortedEventsQuery', () => {
         sort: [
           {
             timefield: {
+              format: 'strict_date_optional_time||epoch_millis',
               order: 'asc',
             },
           },
@@ -320,6 +325,7 @@ describe('buildSortedEventsQuery', () => {
         sort: [
           {
             timefield: {
+              format: 'strict_date_optional_time||epoch_millis',
               order: 'desc',
             },
           },
@@ -368,6 +374,7 @@ describe('buildSortedEventsQuery', () => {
         sort: [
           {
             timefield: {
+              format: 'strict_date_optional_time||epoch_millis',
               order: 'asc',
             },
           },
diff --git a/x-pack/plugins/stack_alerts/common/build_sorted_events_query.ts b/x-pack/plugins/stack_alerts/common/build_sorted_events_query.ts
index 7b92374b8fb33..5253fb8eb74ee 100644
--- a/x-pack/plugins/stack_alerts/common/build_sorted_events_query.ts
+++ b/x-pack/plugins/stack_alerts/common/build_sorted_events_query.ts
@@ -80,6 +80,7 @@ export const buildSortedEventsQuery = ({
         {
           [sortField]: {
             order: sortOrder ?? 'asc',
+            format: 'strict_date_optional_time||epoch_millis',
           },
         },
       ],
diff --git a/x-pack/plugins/stack_alerts/server/rule_types/es_query/lib/fetch_search_source_query.ts b/x-pack/plugins/stack_alerts/server/rule_types/es_query/lib/fetch_search_source_query.ts
index ac9df7a0c0f8d..1e255da508a13 100644
--- a/x-pack/plugins/stack_alerts/server/rule_types/es_query/lib/fetch_search_source_query.ts
+++ b/x-pack/plugins/stack_alerts/server/rule_types/es_query/lib/fetch_search_source_query.ts
@@ -129,7 +129,14 @@ export function updateSearchSource(
 
   const searchSourceChild = searchSource.createChild();
   searchSourceChild.setField('filter', filters as Filter[]);
-  searchSourceChild.setField('sort', [{ [timeFieldName]: SortDirection.desc }]);
+  searchSourceChild.setField('sort', [
+    {
+      [timeFieldName]: {
+        order: SortDirection.desc,
+        format: 'strict_date_optional_time||epoch_millis',
+      },
+    },
+  ]);
   searchSourceChild.setField(
     'aggs',
     buildAggregation({