diff --git a/orbit/utils/general.py b/orbit/utils/general.py
index cb7d15f4..1f4a0a06 100644
--- a/orbit/utils/general.py
+++ b/orbit/utils/general.py
@@ -14,8 +14,15 @@ def update_dict(original_dict, append_dict):
 
 
 def is_ordered_datetime(array):
-    """Returns True if array is ordered and non-repetitive"""
-    return np.all(np.diff(array).astype(float) > 0)
+    """Returns True if array is ordered and non-repetitive
+    Use pandas .diff() method to take care of both tz and non-tz series consistently
+    By default pandas .diff() would generate a NaT for the first period
+    instead of skipping like np.diff. So dropna() to remove.
+    """
+    if isinstance(array, np.ndarray):
+        array = pd.Series(array)
+    diff = array.diff().dropna().values
+    return np.all(diff.astype(float) > 0)
 
 
 def is_even_gap_datetime(array):