BUG: guess_datetime_format doesn't guess format correctly for UTC+1 (pandas-dev#48954)

* BUG: guess_datetime_format doesn't guess format correctly for UTC+1

* :doc: add comment on +9 offset

Co-authored-by: MarcoGorelli <>

Author: MarcoGorelli

pandas/_libs/tslibs/parsing.pyx

@@ -62,6 +62,7 @@ from pandas._libs.tslibs.np_datetime cimport (
     string_to_dts,
 )
 from pandas._libs.tslibs.offsets cimport is_offset_object
+from pandas._libs.tslibs.strptime import array_strptime
 from pandas._libs.tslibs.util cimport (
     get_c_string_buf_and_size,
     is_array,
@@ -958,7 +959,9 @@ def guess_datetime_format(dt_str: str, bint dayfirst=False) -> str | None:
 
     Returns
     -------
-    ret : datetime format string (for `strftime` or `strptime`)
+    str or None : ret
+        datetime format string (for `strftime` or `strptime`),
+        or None if it can't be guessed.
     """
 
     if not isinstance(dt_str, str):
@@ -1079,6 +1082,11 @@ def guess_datetime_format(dt_str: str, bint dayfirst=False) -> str | None:
 
     guessed_format = ''.join(output_format)
 
+    try:
+        array_strptime(np.asarray([dt_str], dtype=object), guessed_format)
+    except ValueError:
+        # Doesn't parse, so this can't be the correct format.
+        return None
     # rebuild string, capturing any inferred padding
     dt_str = ''.join(tokens)
     if parsed_datetime.strftime(guessed_format) == dt_str:

pandas/tests/tslibs/test_parsing.py

@@ -152,25 +152,27 @@ def test_parsers_month_freq(date_str, expected):
         ("2011-12-30T00:00:00", "%Y-%m-%dT%H:%M:%S"),
         ("2011-12-30T00:00:00UTC", "%Y-%m-%dT%H:%M:%S%Z"),
         ("2011-12-30T00:00:00Z", "%Y-%m-%dT%H:%M:%S%z"),
-        ("2011-12-30T00:00:00+9", "%Y-%m-%dT%H:%M:%S%z"),
-        ("2011-12-30T00:00:00+09", "%Y-%m-%dT%H:%M:%S%z"),
+        # The +9 format for offsets is supported by dateutil,
+        # but don't round-trip, see https://github.com/pandas-dev/pandas/issues/48921
+        ("2011-12-30T00:00:00+9", None),
+        ("2011-12-30T00:00:00+09", None),
         ("2011-12-30T00:00:00+090", None),
         ("2011-12-30T00:00:00+0900", "%Y-%m-%dT%H:%M:%S%z"),
         ("2011-12-30T00:00:00-0900", "%Y-%m-%dT%H:%M:%S%z"),
         ("2011-12-30T00:00:00+09:00", "%Y-%m-%dT%H:%M:%S%z"),
-        ("2011-12-30T00:00:00+09:000", "%Y-%m-%dT%H:%M:%S%z"),
-        ("2011-12-30T00:00:00+9:0", "%Y-%m-%dT%H:%M:%S%z"),
+        ("2011-12-30T00:00:00+09:000", None),
+        ("2011-12-30T00:00:00+9:0", None),
         ("2011-12-30T00:00:00+09:", None),
         ("2011-12-30T00:00:00.000000UTC", "%Y-%m-%dT%H:%M:%S.%f%Z"),
         ("2011-12-30T00:00:00.000000Z", "%Y-%m-%dT%H:%M:%S.%f%z"),
-        ("2011-12-30T00:00:00.000000+9", "%Y-%m-%dT%H:%M:%S.%f%z"),
-        ("2011-12-30T00:00:00.000000+09", "%Y-%m-%dT%H:%M:%S.%f%z"),
+        ("2011-12-30T00:00:00.000000+9", None),
+        ("2011-12-30T00:00:00.000000+09", None),
         ("2011-12-30T00:00:00.000000+090", None),
         ("2011-12-30T00:00:00.000000+0900", "%Y-%m-%dT%H:%M:%S.%f%z"),
         ("2011-12-30T00:00:00.000000-0900", "%Y-%m-%dT%H:%M:%S.%f%z"),
         ("2011-12-30T00:00:00.000000+09:00", "%Y-%m-%dT%H:%M:%S.%f%z"),
-        ("2011-12-30T00:00:00.000000+09:000", "%Y-%m-%dT%H:%M:%S.%f%z"),
-        ("2011-12-30T00:00:00.000000+9:0", "%Y-%m-%dT%H:%M:%S.%f%z"),
+        ("2011-12-30T00:00:00.000000+09:000", None),
+        ("2011-12-30T00:00:00.000000+9:0", None),
         ("2011-12-30T00:00:00.000000+09:", None),
         ("2011-12-30 00:00:00.000000", "%Y-%m-%d %H:%M:%S.%f"),
         ("Tue 24 Aug 2021 01:30:48 AM", "%a %d %b %Y %H:%M:%S %p"),