diff --git a/src/CSET/operators/collapse.py b/src/CSET/operators/collapse.py
index 374551ab3..71bb03a8c 100644
--- a/src/CSET/operators/collapse.py
+++ b/src/CSET/operators/collapse.py
@@ -108,7 +108,8 @@ def collapse_by_hour_of_day(
     Collapsing of the cube is around the 'time' coordinate. The coordinates are
     first grouped by the hour of day, and then aggregated by the hour of day
     to create a diurnal cycle. This operator is applicable for both single
-    forecasts and for multiple forecasts.
+    forecasts and for multiple forecasts. The hour used is based on the units of
+    the time coordinate. If the time coordinate is in UTC, hour will be in UTC.
 
     To apply this operator successfully there must only be one time dimension.
     Should a MultiDim exception be raised the user first needs to apply the
diff --git a/tests/operators/test_collapse.py b/tests/operators/test_collapse.py
index 43d5be05e..7b28660a2 100644
--- a/tests/operators/test_collapse.py
+++ b/tests/operators/test_collapse.py
@@ -25,7 +25,7 @@
 def long_forecast() -> iris.cube.Cube:
     """Get long_forecast to run tests on."""
     return iris.load_cube(
-        "tests/test_data/long_forecast_air_temperature.nc", "air_temperature"
+        "tests/test_data/long_forecast_air_temp_fcst_1.nc", "air_temperature"
     )
 
 
diff --git a/tests/test_data/long_forecast_air_temperature.nc b/tests/test_data/long_forecast_air_temp_fcst_1.nc
similarity index 100%
rename from tests/test_data/long_forecast_air_temperature.nc
rename to tests/test_data/long_forecast_air_temp_fcst_1.nc