Spurious "can't determine whether the indexer has a time zone" warnings
**Describe the bug**
When producing time-series plots in the viewer from user-selected geometries, the server outputs this warning repeatedly:

> UserWarning: We can't determine whether the time indexer has a time zone. We will therefore omit the check on whether the time indexer and the variable are incompatible in terms of timezone awareness. This may result in an error when an indexing operation is carried out. If such an error occurs after this warning, make sure that the time indexer has timezone information.

It's followed by the usual pandas / xarray indexing warning:

> FutureWarning: Indexing a timezone-naive DatetimeIndex with a timezone-aware datetime is deprecated and will raise KeyError in a future version.  Use a timezone-naive object instead.

**To Reproduce** 

With any server config specifying a timezone-naive dataset:

```bash
python -m xcube.cli.main --warnings --logfile xcube-server.log --loglevel DETAIL serve -v --config xcube-server.yaml --update-after 1
```

Start viewer, pick a dataset, select some geometries (points, circles, etc.). Observe warnings in server logs.

**Expected behavior**
These warnings should not appear.

**Additional context**
See #605 .

diff --git a/xcube/core/timeseries.py b/xcube/core/timeseries.py
index 97844075..68449cd3 100644
--- a/xcube/core/timeseries.py
+++ b/xcube/core/timeseries.py
@@ -133,10 +133,9 @@ def get_time_series(
         return None
 
     if start_date is not None or end_date is not None:
-        safe_start_date, safe_end_date = \\
-            ensure_time_index_compatible(dataset, (start_date, end_date))
-        # noinspection PyTypeChecker
-        dataset = dataset.sel(time=slice(safe_start_date, safe_end_date))
+        date_slice = slice(start_date, end_date)
+        safe_slice = ensure_time_index_compatible(dataset, date_slice)
+        dataset = dataset.sel(time=safe_slice)
 
     x_name, y_name = grid_mapping.xy_dim_names
     if isinstance(geometry, shapely.geometry.Point):
diff --git a/xcube/util/timeindex.py b/xcube/util/timeindex.py
index e327be4e..46d04473 100644
--- a/xcube/util/timeindex.py
+++ b/xcube/util/timeindex.py
@@ -160,6 +160,7 @@ def _ensure_timestamp_compatible(var: xr.DataArray, time_value: Any,
                     'time indexer', 'time indexer has'
                 )
             )
+            warnings.warn(f'Indexer: {time_value}')
             return time_value
 
     if _has_datetime64_time(var, time_name):
@@ -187,6 +188,7 @@ def _ensure_timestamp_compatible(var: xr.DataArray, time_value: Any,
                     'data in the time co-ordinate have'
                 )
             )
+            warnings.warn(f'First time value: {first_time_value}')
             return time_value
 
     cant_convert_warning_template = (
['xcube/util/timeindex.py', 'xcube/core/timeseries.py']