BUG: inferred resolution with ISO8601 and tzoffset (#56208)

* BUG: inferred resolution with ISO8601 and tzoffset

* GH ref

Author: jbrockmendel

doc/source/whatsnew/v2.2.0.rst

@@ -459,6 +459,7 @@ Datetimelike
 - Bug in :meth:`Index.view` to a datetime64 dtype with non-supported resolution incorrectly raising (:issue:`55710`)
 - Bug in :meth:`Series.dt.round` with non-nanosecond resolution and ``NaT`` entries incorrectly raising ``OverflowError`` (:issue:`56158`)
 - Bug in :meth:`Tick.delta` with very large ticks raising ``OverflowError`` instead of ``OutOfBoundsTimedelta`` (:issue:`55503`)
+- Bug in :meth:`Timestamp.unit` being inferred incorrectly from an ISO8601 format string with minute or hour resolution and a timezone offset (:issue:`56208`)
 - Bug in ``.astype`` converting from a higher-resolution ``datetime64`` dtype to a lower-resolution ``datetime64`` dtype (e.g. ``datetime64[us]->datetim64[ms]``) silently overflowing with values near the lower implementation bound (:issue:`55979`)
 - Bug in adding or subtracting a :class:`Week` offset to a ``datetime64`` :class:`Series`, :class:`Index`, or :class:`DataFrame` column with non-nanosecond resolution returning incorrect results (:issue:`55583`)
 - Bug in addition or subtraction of :class:`BusinessDay` offset with ``offset`` attribute to non-nanosecond :class:`Index`, :class:`Series`, or :class:`DataFrame` column giving incorrect results (:issue:`55608`)

pandas/_libs/src/vendored/numpy/datetime/np_datetime_strings.c

@@ -364,6 +364,7 @@ int parse_iso_8601_datetime(const char *str, int len, int want_exc,
     goto parse_error;
   }
   out->hour = (*substr - '0');
+  bestunit = NPY_FR_h;
   ++substr;
   --sublen;
   /* Second digit optional */
@@ -425,6 +426,7 @@ int parse_iso_8601_datetime(const char *str, int len, int want_exc,
   }
   /* First digit required */
   out->min = (*substr - '0');
+  bestunit = NPY_FR_m;
   ++substr;
   --sublen;
   /* Second digit optional if there was a separator */

pandas/tests/scalar/timestamp/test_constructors.py

@@ -457,6 +457,13 @@ def test_constructor_str_infer_reso(self):
         assert ts == Timestamp("01-01-2013T00:00:00.000000002+0000")
         assert ts.unit == "ns"
 
+        # GH#56208 minute reso through the ISO8601 path with tz offset
+        ts = Timestamp("2020-01-01 00:00+00:00")
+        assert ts.unit == "s"
+
+        ts = Timestamp("2020-01-01 00+00:00")
+        assert ts.unit == "s"
+
 
 class TestTimestampConstructors:
     def test_weekday_but_no_day_raises(self):