diff --git a/flox/xarray.py b/flox/xarray.py
index acd3f2d6c..487850ca0 100644
--- a/flox/xarray.py
+++ b/flox/xarray.py
@@ -374,8 +374,8 @@ def wrapper(array, *by, func, skipna, core_dims, **kwargs):
                 # xarray always uses np.datetime64[ns] for np.datetime64 data
                 dtype = "timedelta64[ns]"
                 array = datetime_to_numeric(array, offset)
-            elif _contains_cftime_datetimes(array):
-                offset = min(array)
+            elif is_cftime:
+                offset = array.min()
                 array = datetime_to_numeric(array, offset, datetime_unit="us")
 
         result, *groups = groupby_reduce(array, *by, func=func, **kwargs)
diff --git a/flox/xrutils.py b/flox/xrutils.py
index 45cf45eec..958bd3976 100644
--- a/flox/xrutils.py
+++ b/flox/xrutils.py
@@ -157,7 +157,7 @@ def datetime_to_numeric(array, offset=None, datetime_unit=None, dtype=float):
         if array.dtype.kind in "Mm":
             offset = _datetime_nanmin(array)
         else:
-            offset = min(array)
+            offset = array.min()
 
     # Compute timedelta object.
     # For np.datetime64, this can silently yield garbage due to overflow.