Fix %I/%-I strftime directives to use the 12-hour clock

The %I and %-I directives mapped straight to the hour attribute (0-23),
making them identical to %H. Per the strftime specification (and the
'12' fallback already present) they are the 12-hour clock, so e.g. 1pm
rendered as '13' instead of '01' and midnight as '00' instead of '12'.

Add an hour12 property (hour % 12, with 0 mapped to 12) and point both
directives at it. A plain date (no time) still falls back to '12'.

Author: gaoflow

CHANGELOG.md

@@ -1,5 +1,11 @@
 # Changelog
 
+## [Unreleased]
+
+### Fixed
+
+* `%I` and `%-I` now render the 12-hour clock instead of duplicating `%H`
+
 ## [5.3.0] - 2026-05-19
 
 ## Add

jdatetime/__init__.py

@@ -44,8 +44,8 @@
     '%f': ('_strftime_get_attr_value', {'attr': 'microsecond', 'fmt': '%06.d', 'fb': '000000'}),
     '%H': ('_strftime_get_attr_value', {'attr': 'hour', 'fmt': '%02.d', 'fb': '00'}),
     '%-H': ('_strftime_get_attr_value', {'attr': 'hour', 'fmt': '%d', 'fb': '0'}),
-    '%I': ('_strftime_get_attr_value', {'attr': 'hour', 'fmt': '%02.d', 'fb': '12'}),
-    '%-I': ('_strftime_get_attr_value', {'attr': 'hour', 'fmt': '%d', 'fb': '12'}),
+    '%I': ('_strftime_get_attr_value', {'attr': 'hour12', 'fmt': '%02.d', 'fb': '12'}),
+    '%-I': ('_strftime_get_attr_value', {'attr': 'hour12', 'fmt': '%d', 'fb': '12'}),
     '%M': ('_strftime_get_attr_value', {'attr': 'minute', 'fmt': '%02.d', 'fb': '00'}),
     '%-M': ('_strftime_get_attr_value', {'attr': 'minute', 'fmt': '%d', 'fb': '0'}),
     '%S': ('_strftime_get_attr_value', {'attr': 'second', 'fmt': '%02.d', 'fb': '00'}),
@@ -875,6 +875,12 @@ def fromordinal(ordinal: int) -> datetime:
     def hour(self) -> int:
         return self.__time.hour
 
+    @property
+    def hour12(self) -> int:
+        """Return the hour on a 12-hour clock (1..12), used by the %I directive."""
+        hour = self.hour % 12
+        return 12 if hour == 0 else hour
+
     @property
     def minute(self) -> int:
         return self.__time.minute

tests/test_jdatetime.py

@@ -211,6 +211,31 @@ def dst(self, dt):
         dt = jdatetime.datetime(1389, 2, 17, 19, 10, 2, tzinfo=teh)
         self.assertEqual(dt.strftime('%Z %z'), 'IRDT +0330')
 
+    def test_strftime_I_directive_uses_twelve_hour_clock(self):
+        import datetime as _std
+
+        # %I and %-I must render the 12-hour clock (1..12), matching stdlib.
+        for hour in range(24):
+            with self.subTest(hour=hour):
+                jd = jdatetime.datetime(1400, 1, 1, hour, 5, 0)
+                std = _std.datetime(2020, 1, 1, hour, 5, 0)
+                self.assertEqual(jd.strftime('%I'), std.strftime('%I'))
+                self.assertEqual(jd.strftime('%-I'), std.strftime('%-I'))
+                self.assertEqual(jd.hour12, int(std.strftime('%I')))
+                # %I must be consistent with the (correct) %p directive:
+                # reconstructing the 24-hour value from (%p, %I) round-trips.
+                twelve = int(jd.strftime('%I'))
+                base = 0 if twelve == 12 else twelve
+                reconstructed = base + (12 if jd.strftime('%p') == 'PM' else 0)
+                self.assertEqual(reconstructed, hour)
+        # Spot-check the values that the previous 24-hour behaviour got wrong.
+        self.assertEqual(jdatetime.datetime(1400, 1, 1, 0, 0).strftime('%I'), '12')
+        self.assertEqual(jdatetime.datetime(1400, 1, 1, 13, 0).strftime('%I'), '01')
+        self.assertEqual(jdatetime.datetime(1400, 1, 1, 23, 0).strftime('%I'), '11')
+        self.assertEqual(jdatetime.datetime(1400, 1, 1, 13, 0).strftime('%-I'), '1')
+        # A plain date (no time component) is midnight -> 12.
+        self.assertEqual(jdatetime.date(1400, 1, 1).strftime('%I'), '12')
+
     def test_strftime_fa_locale_uses_short_month_names_for_b_directive(self):
         tests = [
             (1, 'فرو', 'فروردین'),