diff --git a/apps/time_entries/api/serializers.py b/apps/time_entries/api/serializers.py
index 4e526da..01b987b 100644
--- a/apps/time_entries/api/serializers.py
+++ b/apps/time_entries/api/serializers.py
@@ -93,7 +93,7 @@ class TimeEntryCreateSerializer(serializers.Serializer):
     """
     workspace_id = serializers.UUIDField()
     project_id = serializers.UUIDField(required=False, allow_null=True)
-    start_time = serializers.DateTimeField()
+    start_time = serializers.DateTimeField(required=False)
     end_time = serializers.DateTimeField(required=False, allow_null=True)
     description = serializers.CharField(required=False, allow_blank=True, default="")
     tags = serializers.ListField(child=serializers.UUIDField(), required=False)
@@ -102,6 +102,12 @@ class TimeEntryCreateSerializer(serializers.Serializer):
     def validate(self, attrs):
         user = self.context.get("request").user if self.context.get("request") else None
         workspace_id = attrs.get("workspace_id")
+        start_time = attrs.get("start_time")
+        end_time = attrs.get("end_time")
+
+        if end_time is not None and start_time is None:
+            raise serializers.ValidationError({"start_time": "Start time is required when end time is provided."})
+
         project_id = attrs.pop("project_id", serializers.empty)
         if project_id is not serializers.empty:
             if project_id is None:
diff --git a/apps/time_entries/services/time_entries.py b/apps/time_entries/services/time_entries.py
index fa6c4c2..01163af 100644
--- a/apps/time_entries/services/time_entries.py
+++ b/apps/time_entries/services/time_entries.py
@@ -22,24 +22,29 @@ def _verify_workspace_access(user, workspace_id):
         raise PermissionDenied("You do not have access to this workspace.")
 
 
-def create_time_entry(user, workspace_id, start_time, end_time=None, project=None, tags=None, description="", is_billable=False):
+def create_time_entry(user, workspace_id, start_time=None, end_time=None, project=None, tags=None, description="", is_billable=False):
     """
     Creates a new time entry. If end_time is None, it acts as a running timer.
     """
     _verify_workspace_access(user, workspace_id)
 
-    if not end_time:
-        has_running_timer = TimeEntry.objects.filter(
-            workspace_id=workspace_id, 
-            user=user, 
-            end_time__isnull=True, 
-            is_deleted=False
-        ).exists()
-        if has_running_timer:
-            raise ValidationError({"non_field_errors": "You already have a running timer in this workspace."})
-
-    if start_time and end_time and start_time >= end_time:
-        raise ValidationError({"end_time": "End time must be strictly after start time."})
+    if not end_time:
+        has_running_timer = TimeEntry.objects.filter(
+            workspace_id=workspace_id, 
+            user=user, 
+            end_time__isnull=True, 
+            is_deleted=False
+        ).exists()
+        if has_running_timer:
+            raise ValidationError({"non_field_errors": "You already have a running timer in this workspace."})
+
+    if start_time is None:
+        if end_time is not None:
+            raise ValidationError({"start_time": "Start time is required when end time is provided."})
+        start_time = timezone.now()
+
+    if start_time and end_time and start_time >= end_time:
+        raise ValidationError({"end_time": "End time must be strictly after start time."})
 
     if project and project.workspace_id != workspace_id:
         raise ValidationError({"project": "Project must belong to the same workspace."})
diff --git a/apps/time_entries/tests/test_services.py b/apps/time_entries/tests/test_services.py
index 241ccc4..05925c1 100644
--- a/apps/time_entries/tests/test_services.py
+++ b/apps/time_entries/tests/test_services.py
@@ -47,6 +47,18 @@ class TimeEntryServiceTests(TestCase):
         self.assertIsNotNone(stopped_entry.end_time)
         self.assertIsNotNone(stopped_entry.duration)
 
+    def test_create_running_time_entry_defaults_start_time_to_server_now(self):
+        before = timezone.now()
+        entry = create_time_entry(
+            user=self.user,
+            workspace_id=self.workspace.id,
+        )
+        after = timezone.now()
+
+        self.assertIsNone(entry.end_time)
+        self.assertGreaterEqual(entry.start_time, before)
+        self.assertLessEqual(entry.start_time, after)
+
     def test_update_time_entry_preserves_deleted_project_and_tags(self):
         project = Project.objects.create(workspace=self.workspace, name="Deleted project")
         tag = Tag.objects.create(
diff --git a/apps/time_entries/tests/test_views.py b/apps/time_entries/tests/test_views.py
index a4782a6..6116241 100644
--- a/apps/time_entries/tests/test_views.py
+++ b/apps/time_entries/tests/test_views.py
@@ -19,6 +19,28 @@ def make_aware(year, month, day, hour=9, minute=0, second=0):
 
 
 class TimeEntryViewTests(APITestCase):
+    def test_create_running_time_entry_without_start_time_uses_server_time(self):
+        user = User.objects.create_user(mobile="09125555555", password="secret123")
+        workspace = Workspace.objects.create(name="Core", owner=user)
+
+        self.client.force_authenticate(user=user)
+        before = timezone.now()
+        response = self.client.post(
+            "/api/time-entries/",
+            {
+                "workspace_id": str(workspace.id),
+                "description": "Running work",
+            },
+            format="json",
+        )
+        after = timezone.now()
+
+        self.assertEqual(response.status_code, 201)
+        entry = TimeEntry.objects.get(id=response.data["id"])
+        self.assertIsNone(entry.end_time)
+        self.assertGreaterEqual(entry.start_time, before)
+        self.assertLessEqual(entry.start_time, after)
+
     def test_time_entry_list_returns_grouped_payload_for_ended_entries(self):
         user = User.objects.create_user(mobile="09126666666", password="secret123")
         workspace = Workspace.objects.create(name="Core", owner=user)