Validate that --event-time-start and --event-time-end are mutuall…

…y present

core/dbt/cli/flags.py

@@ -361,7 +361,23 @@ def _validate_event_time_configs(self) -> None:
             getattr(self, "EVENT_TIME_END") if hasattr(self, "EVENT_TIME_END") else None
         )
 
-        if event_time_start is not None and event_time_end is not None:
+        # only do validations if at least one of `event_time_start` or `event_time_end` are specified
+        if event_time_start is not None or event_time_end is not None:
+
+            # These `ifs`, combined with the parent `if` make it so that `event_time_start` and
+            # `event_time_end` are mutually required
+            if event_time_start is None:
+                raise DbtUsageException(
+                    "The flag `--event-time-end` was specified, but `--event-time-start` was not. "
+                    "When specifying `--event-time-end`, `--event-time-start` must also be present."
+                )
+            if event_time_end is None:
+                raise DbtUsageException(
+                    "The flag `--event-time-start` was specified, but `--event-time-end` was not. "
+                    "When specifying `--event-time-start`, `--event-time-end` must also be present."
+                )
+
+            # This `if` just is a sanity check that `event_time_start` is before `event_time_end`
             if event_time_start >= event_time_end:
                 raise DbtUsageException(
                     "Value for `--event-time-start` must be less than `--event-time-end`"

tests/functional/cli/test_option_interaction_validations.py

@@ -29,3 +29,21 @@ def test_option_combo(self, project, event_time_start, event_time_end, expect_pa
                 in e.__str__()
             )
             assert not expect_pass
+
+
+class TestEventTimeEndEventTimeStartMutuallyRequired:
+    @pytest.mark.parametrize(
+        "specified,missing",
+        [
+            ("--event-time-start", "--event-time-end"),
+            ("--event-time-end", "--event-time-start"),
+        ],
+    )
+    def test_option_combo(self, project, specified, missing):
+        try:
+            run_dbt(["build", specified, "2024-10-01"])
+            assert False, f"An error should have been raised for missing `{missing}` flag"
+        except Exception as e:
+            assert (
+                f"When specifying `{specified}`, `{missing}` must also be present." in e.__str__()
+            )