@@ -1112,20 +1112,35 @@ def test_transitions_in_adjacent_year(self):
         # rule evaluation. Cover starts and ends, either sign of DST, and
         # rules displaced by both UTC offsets and extended transition hours.
         cases = (
+            # Positive DST starts on January 1 locally, but December 31 UTC.
             ("STD-2DST-3,J1/0,J200/0", datetime(2100, 12, 31, 22), 2, 3),
+            # Positive DST starts in January from the previous year's J365.
             ("STD2DST1,J365/26,J200/0", datetime(2101, 1, 1, 4), -2, -1),
+            # Negative DST starts on January 1 locally, but December 31 UTC.
             ("STD-2DST-1,J1/0,J200/0", datetime(2100, 12, 31, 22), 2, 1),
+            # Negative DST starts in January from the previous year's J365.
             ("STD2DST3,J365/26,J200/0", datetime(2101, 1, 1, 4), -2, -3),
+            # Positive DST ends on January 1 locally, but December 31 UTC.
             ("STD-2DST-3,J200/0,J1/0", datetime(2100, 12, 31, 21), 3, 2),
+            # Positive DST ends in January from the previous year's J365.
             ("STD2DST1,J200/0,J365/26", datetime(2101, 1, 1, 3), -1, -2),
+            # Negative DST ends on January 1 locally, but December 31 UTC.
             ("STD-2DST-1,J200/0,J1/0", datetime(2100, 12, 31, 23), 1, 2),
+            # Negative DST ends in January from the previous year's J365.
             ("STD2DST3,J200/0,J365/26", datetime(2101, 1, 1, 5), -3, -2),
+            # The most negative transition hour moves J1 into December.
             ("STD0DST-1,J1/-167,J200/0", datetime(2099, 12, 25, 1), 0, 1),
+            # The largest transition hour moves J365 into January.
             ("STD0DST-1,J200/0,J365/167", datetime(2101, 1, 6, 22), 1, 0),
+            # A month/week rule from year 0 supplies a transition in year 1.
             ("STD0DST-1,M12.5.0/167,M6.1.0/0", datetime(1, 1, 6, 23), 0, 1),
+            # A year-10000 rule supplies a transition within datetime's range.
             ("STD0DST-1,J1/-167,J200/0", datetime(9999, 12, 25, 1), 0, 1),
+            # A season starting in rule year 2023 ends in January 2025.
             ("STD0DST-1,J365/167,J365/166", datetime(2025, 1, 6, 21), 1, 0),
+            # The next season starts two hours after the preceding one ends.
             ("STD0DST-1,J365/167,J365/166", datetime(2025, 1, 6, 23), 0, 1),
+            # The same two-rule-year lookback must work through year -1.
             ("STD0DST-1,J365/167,J365/166", datetime(1, 1, 6, 21), 1, 0),
         )
         for tzstr, transition, old_hours, new_hours in cases:
 
@@ -11,6 +11,17 @@
 EPOCH = datetime(1970, 1, 1)
 EPOCHORDINAL = datetime(1970, 1, 1).toordinal()
 
+_SECONDS_PER_DAY = 24 * 60 * 60
+# Parsed transition times are less than 168 hours and explicit UTC offsets
+# less than 25 hours (26 with the default one-hour DST adjustment). Offset
+# differences are less than 50 hours. Their sum is less than 244 hours, so
+# an 11-day margin also covers looking back across a fold.
+_TZSTR_YEAR_BOUNDARY_MARGIN = 11 * _SECONDS_PER_DAY
+# Use the common-year length; in leap years this leaves an extra day of margin.
+_TZSTR_FAST_PATH_END = 365 * _SECONDS_PER_DAY - _TZSTR_YEAR_BOUNDARY_MARGIN
+# Each rule's day-of-year can vary by up to seven days between years.
+_TZSTR_RULE_ORDER_MARGIN = 2 * 7 * _SECONDS_PER_DAY
+
 # It is relatively expensive to construct new timedelta objects, and in most
 # cases we're looking at the same deltas, like integer numbers of hours, etc.
 # To improve speed and memory use, we'll keep a dictionary with references
@@ -474,15 +485,14 @@ def _dst_period(self, year):
     def _get_trans_info(self, ts, year, fold):
         """Get the information about the current transition - tti"""
 
-        # Even the largest parsed transition time (<168h), UTC offset
-        # (<25h), and fold (<50h) cannot reach 11 days from a year boundary.
-        year_start = (_post_epoch_days_before_year(year) + 1) * 86400
-        if year_start + 11 * 86400 <= ts < year_start + 354 * 86400:
+        year_start = (_post_epoch_days_before_year(year) + 1) * _SECONDS_PER_DAY
+        if (year_start + _TZSTR_YEAR_BOUNDARY_MARGIN <= ts
+                < year_start + _TZSTR_FAST_PATH_END):
             start, end = self.transitions(year)
             # Each rule's day-of-year can move by up to seven days between
             # years. Nearby rules can reverse order, allowing a previous
             # year's southern season to overlap this year's northern one.
-            if abs(end - start - self.dst_diff) > 14 * 86400:
+            if abs(end - start - self.dst_diff) > _TZSTR_RULE_ORDER_MARGIN:
                 if fold == (self.dst_diff >= 0):
                     end -= self.dst_diff
                 else:
@@ -518,12 +528,13 @@ def _get_trans_info(self, ts, year, fold):
         return self.std
 
     def _get_trans_info_fromutc(self, ts, year):
-        year_start = (_post_epoch_days_before_year(year) + 1) * 86400
-        if year_start + 11 * 86400 <= ts < year_start + 354 * 86400:
+        year_start = (_post_epoch_days_before_year(year) + 1) * _SECONDS_PER_DAY
+        if (year_start + _TZSTR_YEAR_BOUNDARY_MARGIN <= ts
+                < year_start + _TZSTR_FAST_PATH_END):
             start, end = self.transitions(year)
             start -= self.std.utcoff.total_seconds()
             end -= self.dst.utcoff.total_seconds()
-            if abs(end - start) > 14 * 86400:
+            if abs(end - start) > _TZSTR_RULE_ORDER_MARGIN:
                 before = ts - abs(self.dst_diff)
                 if start < end:
                     isdst = start <= ts < end