diff --git a/sup3r/utilities/utilities.py b/sup3r/utilities/utilities.py
index c79467fb3..ebfb07a50 100644
--- a/sup3r/utilities/utilities.py
+++ b/sup3r/utilities/utilities.py
@@ -22,7 +22,11 @@ def preprocess_datasets(dset):
     """Standardization preprocessing applied before datasets are concatenated
     by ``xr.open_mfdataset``"""
     if 'time' in dset and dset.time.size > 1:
-        ti = dset.time.astype(int)
+        if 'time' in dset.indexes and hasattr(
+            dset.indexes['time'], 'to_datetimeindex'
+        ):
+            dset['time'] = dset.indexes['time'].to_datetimeindex()
+        ti = dset['time'].astype(int)
         dset['time'] = ti
     if 'latitude' in dset.dims:
         dset = dset.swap_dims({'latitude': 'south_north'})
diff --git a/tests/utilities/test_era_downloader.py b/tests/utilities/test_era_downloader.py
index 5b79109c1..2afbc137b 100644
--- a/tests/utilities/test_era_downloader.py
+++ b/tests/utilities/test_era_downloader.py
@@ -98,7 +98,11 @@ def test_era_dl_year(tmpdir_factory):
         yearly_file_pattern=yearly_file_pattern,
         max_workers=1,
         combine_all_files=True,
-        res_kwargs={'compat': 'override', 'engine': 'netcdf4'},
+        res_kwargs={
+            'compat': 'override',
+            'engine': 'netcdf4',
+            'concat_dim': 'time',
+        },
     )
 
     combined_file = yearly_file_pattern.replace('_{var}_', '').format(