gh-159712: Fix zero-based day rules in zoneinfo

Numeric TZ day rules count January 1 as day zero, but both implementations
were using the same year baseline as one-based Julian rules. Account for
the difference so numeric rules no longer transition one day early.

Add local-time and UTC conversion cases for numeric rules, including gaps,
folds, leap days and the first and last days of the year.

Author: pganssle

Lib/test/test_zoneinfo/test_zoneinfo.py

@@ -1658,6 +1658,29 @@ def _add():
                 (datetime(2012, 11, 4, 2, 15, 45), AAA, NORMAL),
             )
 
+        @call
+        def _add():
+            # Numeric day rules count from January 1, including leap days.
+            STD = ZoneOffset("STD", ZERO)
+            DST = ZoneOffset("DST", ONE_H, ONE_H)
+
+            for day in (0, 10, 59, 60, 364, 365):
+                tzstr = f"STD0DST,{day}/2,{day}/14"
+                day_cases = []
+                for year in (2019, 2020):
+                    date = datetime(year, 1, 1) + timedelta(days=day)
+                    day_cases.extend((
+                        (date.replace(hour=1, minute=59), STD, NORMAL),
+                        (date.replace(hour=2, fold=0), STD, GAP),
+                        (date.replace(hour=2, fold=1), DST, GAP),
+                        (date.replace(hour=3), DST, NORMAL),
+                        (date.replace(hour=12, minute=59), DST, NORMAL),
+                        (date.replace(hour=13, fold=0), DST, FOLD),
+                        (date.replace(hour=13, fold=1), STD, FOLD),
+                        (date.replace(hour=14), STD, NORMAL),
+                    ))
+                cases[tzstr] = day_cases
+
         cls.test_cases = cases
 
 

Lib/zoneinfo/_zoneinfo.py

@@ -583,6 +583,9 @@ def year_to_epoch(self, year):
         d = self.d
         if self.julian and d >= 59 and calendar.isleap(year):
             d += 1
+        elif not self.julian:
+            # Numeric day rules are zero-based, unlike Julian day rules.
+            d += 1
 
         epoch = (days_before_year + d) * 86400
         epoch += self.hour * 3600 + self.minute * 60 + self.second

Misc/NEWS.d/next/Library/2026-10-19-12-00-00.gh-issue-159712.zero-based-days.rst

@@ -0,0 +1,2 @@
+Fix :class:`zoneinfo.ZoneInfo` evaluating numeric day-of-year rules in TZ
+strings one day early.

Modules/_zoneinfo.c

@@ -1439,6 +1439,10 @@ dayrule_year_to_timestamp(TransitionRuleType *base_self, int year)
     if (self->julian && day >= 59 && is_leap_year(year)) {
         day += 1;
     }
+    else if (!self->julian) {
+        // Numeric day rules are zero-based, unlike Julian day rules.
+        day += 1;
+    }
 
     return (days_before_year + day) * 86400 + (int64_t)self->hour * 3600 +
            (int64_t)self->minute * 60 + self->second;