diff --git a/src/jsonata/functions.py b/src/jsonata/functions.py
index 584ada7..6a5913e 100644
--- a/src/jsonata/functions.py
+++ b/src/jsonata/functions.py
@@ -2105,7 +2105,10 @@ def datetime_to_millis(timestamp: Optional[str], picture: Optional[str]) -> Opti
                 dt = datetime.datetime.strptime(timestamp, "%Y")
             else:
                 dt = datetime.datetime.fromisoformat(timestamp)
-            dt = dt.replace(tzinfo=datetime.timezone.utc)
+            if dt.tzinfo is None:
+                dt = dt.replace(tzinfo=datetime.timezone.utc)
+            else:
+                dt = dt.astimezone(datetime.timezone.utc)
             return int(dt.timestamp() * 1000)
             # try:
             #     size = len(timestamp)