diff --git a/src/time_zone_info.cc b/src/time_zone_info.cc
index 8820762..fe5dd28 100644
--- a/src/time_zone_info.cc
+++ b/src/time_zone_info.cc
@@ -269,6 +269,13 @@ inline civil_second YearShift(const civil_second& cs, year_t shift) {
 bool TimeZoneInfo::GetTransitionType(std::int_fast32_t utc_offset, bool is_dst,
                                      const std::string& abbr,
                                      std::uint_least8_t* index) {
+  // Keep synthesized types within a day of UTC, matching the bound the reader
+  // enforces on file-supplied types (see Load). ParsePosixSpec() accepts an
+  // std/dst offset field up to 24:59:59, so a future spec can otherwise reach
+  // here with an out-of-range offset the TZif path would have rejected.
+  if (utc_offset <= -kSecsPerDay || utc_offset >= kSecsPerDay) {
+    return false;
+  }
   std::size_t type_index = 0;
   std::size_t abbr_index = abbreviations_.size();
   for (; type_index != transition_types_.size(); ++type_index) {
diff --git a/src/time_zone_lookup_test.cc b/src/time_zone_lookup_test.cc
index a386879..60dc15d 100644
--- a/src/time_zone_lookup_test.cc
+++ b/src/time_zone_lookup_test.cc
@@ -1080,6 +1080,14 @@ std::unique_ptr<ZoneInfoSource> ExtendedTestFactory(
     return std::unique_ptr<ZoneInfoSource>(new StringZoneInfoSource(
         MakeExtendedTzif(0, -5 * 3600, "EST", "EST5EDT,M3.2.0,M11.1.0")));
   }
+  if (name == "test:ExtendedOversizeOffset") {
+    // The DST offset field 24:30:00 (88200s) is more than a day from UTC.
+    // The reader rejects such offsets in file-supplied types, so the type
+    // synthesized from the POSIX footer must be rejected too.
+    return std::unique_ptr<ZoneInfoSource>(new StringZoneInfoSource(
+        MakeExtendedTzif(0, -5 * 3600, std::string{"EST", 4},
+                         "EST5EDT24:30:00,M3.2.0,M11.1.0")));
+  }
   return fallback(name);
 }
 
@@ -1122,6 +1130,19 @@ TEST(TimeZoneEdgeCase, ExtendedOverlappingRules) {
   cctz_extension::zone_info_source_factory = prev_factory;
 }
 
+// A POSIX footer whose std/dst offset field is a day or more from UTC yields
+// a transition type the TZif reader would reject; loading must fail rather
+// than build a zone with an out-of-range offset.
+TEST(TimeZoneEdgeCase, ExtendedOversizeOffset) {
+  auto prev_factory = cctz_extension::zone_info_source_factory;
+  cctz_extension::zone_info_source_factory = ExtendedTestFactory;
+
+  time_zone tz;
+  EXPECT_FALSE(load_time_zone("test:ExtendedOversizeOffset", &tz));
+
+  cctz_extension::zone_info_source_factory = prev_factory;
+}
+
 // Looking up the maximum time in an extended zone must fold back through the
 // 400-year cycle without overflowing when BreakTime() computes the shift.
 TEST(TimeZoneEdgeCase, ExtendedFarFuture) {