diff --git a/include/nocal/storage/ics_store.hpp b/include/nocal/storage/ics_store.hpp
index 636a4c6..c2f741a 100644
--- a/include/nocal/storage/ics_store.hpp
+++ b/include/nocal/storage/ics_store.hpp
@@ -27,6 +27,15 @@ private:
     friend class IcsStore;
 };
 
+// Embedded VTIMEZONE definition preserved for round-trip serialization.
+struct VTimezoneDefinition {
+    std::string tzid;
+    // Complete raw content between BEGIN:VTIMEZONE and END:VTIMEZONE (exclusive
+    // of the framing lines). Empty when the definition was not parsed from a
+    // source calendar.
+    std::string raw;
+};
+
 struct LoadResult {
     std::vector<Event> events;
     std::vector<std::string> warnings;
@@ -36,6 +45,9 @@ struct LoadResult {
     // Exact bytes that produced this result. Copies share immutable storage and
     // can be supplied to save/restore as an optimistic concurrency token.
     FileRevision revision;
+    // Embedded VTIMEZONE definitions from the source calendar, preserved for
+    // round-trip serialization. Populated only by load().
+    std::vector<VTimezoneDefinition> vtimezones;
 };
 
 class IcsStore {
@@ -49,9 +61,11 @@ public:
     // requests durable filesystem flushes. I/O failures before replacement are
     // reported as std::runtime_error. If expected is present, the destination
     // must still contain the exact bytes represented by that revision.
+    // Supply vtimezones to preserve embedded timezone definitions.
     static FileRevision save(
         const std::filesystem::path& path, std::span<const Event> events,
-        std::optional<FileRevision> expected = std::nullopt);
+        std::optional<FileRevision> expected = std::nullopt,
+        std::span<const VTimezoneDefinition> vtimezones = {});
 
     // Restores the exact bytes in <path>.bak without changing the backup.
     static FileRevision restore_backup(
diff --git a/src/storage/ics_store.cpp b/src/storage/ics_store.cpp
index 9b8b91f..f02d8b9 100644
--- a/src/storage/ics_store.cpp
+++ b/src/storage/ics_store.cpp
@@ -55,6 +55,16 @@ struct PendingEvent {
     std::vector<PendingProperty> exception_dates;
 };
 
+// Accumulated VTIMEZONE definitions from the source calendar (populated during load).
+std::vector<VTimezoneDefinition> parsed_vtimezones;
+bool inside_vtimezone = false;
+bool vtimezone_has_errors = false;
+
+[[nodiscard]] bool vtimezone_defined(std::string_view tzid) noexcept {
+    return std::any_of(parsed_vtimezones.begin(), parsed_vtimezones.end(),
+        [&tzid](const VTimezoneDefinition& tz) { return tz.tzid == tzid; });
+}
+
 [[nodiscard]] std::string upper(std::string_view value) {
     std::string result(value);
     std::transform(result.begin(), result.end(), result.begin(), [](unsigned char ch) {
@@ -358,6 +368,10 @@ void mark_unsafe_to_rewrite(LoadResult& result) {
     }
 }
 
+[[nodiscard]] bool is_unknown_tzid_warning(std::string_view warning) noexcept {
+    return warning.find("unknown TZID=") != std::string::npos;
+}
+
 [[nodiscard]] std::vector<std::string_view> split(
     std::string_view value, char delimiter) {
     std::vector<std::string_view> parts;
@@ -586,17 +600,22 @@ template <typename Integer>
 }
 
 [[nodiscard]] std::string format_zoned_time(TimePoint value, std::string_view zone_name) {
-    const time_zone* zone = locate_zone(zone_name);
-    const local_seconds local = zone->to_local(floor<seconds>(value));
-    const local_days date = floor<days>(local);
-    const year_month_day ymd{date};
-    const hh_mm_ss time{local - date};
-    return four_digits(static_cast<int>(ymd.year()))
-        + two_digits(static_cast<int>(static_cast<unsigned>(ymd.month())))
-        + two_digits(static_cast<int>(static_cast<unsigned>(ymd.day()))) + "T"
-        + two_digits(static_cast<int>(time.hours().count()))
-        + two_digits(static_cast<int>(time.minutes().count()))
-        + two_digits(static_cast<int>(time.seconds().count()));
+    try {
+        const time_zone* zone = locate_zone(zone_name);
+        const local_seconds local = zone->to_local(floor<seconds>(value));
+        const local_days date = floor<days>(local);
+        const year_month_day ymd{date};
+        const hh_mm_ss time{local - date};
+        return four_digits(static_cast<int>(ymd.year()))
+            + two_digits(static_cast<int>(static_cast<unsigned>(ymd.month())))
+            + two_digits(static_cast<int>(static_cast<unsigned>(ymd.day()))) + "T"
+            + two_digits(static_cast<int>(time.hours().count()))
+            + two_digits(static_cast<int>(time.minutes().count()))
+            + two_digits(static_cast<int>(time.seconds().count()));
+    } catch (const std::runtime_error&) {
+        // Custom TZID not known to the system; fall back to local formatting.
+        return format_floating_time(value);
+    }
 }
 
 [[nodiscard]] std::string format_event_time(const Event& event, TimePoint value) {
@@ -704,11 +723,25 @@ void write_text_property(std::ostream& output, std::string_view name, std::strin
     }
 }
 
-void serialize_calendar(std::ostream& output, std::span<const Event> events) {
+void serialize_calendar(std::ostream& output, std::span<const Event> events,
+                        std::span<const VTimezoneDefinition> vtimezones) {
     write_folded(output, "BEGIN:VCALENDAR");
     write_folded(output, "VERSION:2.0");
     write_folded(output, "PRODID:-//Nomarchy Linux//nocal 1.0//EN");
     write_folded(output, "CALSCALE:GREGORIAN");
+    // Preserve embedded VTIMEZONE definitions for round-trip safety.
+    for (const auto& vt : vtimezones) {
+        if (vt.tzid.empty()) continue;
+        write_folded(output, "BEGIN:VTIMEZONE");
+        write_folded(output, "TZID:" + vt.tzid);
+        if (!vt.raw.empty()) {
+            std::istringstream raw_stream(vt.raw);
+            for (const auto& line : unfold(raw_stream)) {
+                write_folded(output, line);
+            }
+        }
+        write_folded(output, "END:VTIMEZONE");
+    }
     for (const Event& event : events) {
         write_folded(output, "BEGIN:VEVENT");
         write_folded(output, "UID:" + escape_text(event.uid));
@@ -1058,7 +1091,8 @@ void validate_recurrence(const Event& event) {
     }
 }
 
-void validate_event_for_save(const Event& event) {
+void validate_event_for_save(
+    const Event& event, std::span<const VTimezoneDefinition> vtimezones) {
     if (event.uid.empty()) {
         throw std::invalid_argument("cannot save an event with an empty UID");
     }
@@ -1103,9 +1137,13 @@ void validate_event_for_save(const Event& event) {
             try {
                 (void)locate_zone(event.time_zone);
             } catch (const std::runtime_error&) {
-                throw std::invalid_argument(
-                    "cannot save zoned event '" + event.uid + "' with unknown TZID '"
-                    + event.time_zone + "'");
+                bool defined_by_vtimezone = std::any_of(vtimezones.begin(), vtimezones.end(),
+                    [&event](const VTimezoneDefinition& tz) { return tz.tzid == event.time_zone; });
+                if (!defined_by_vtimezone) {
+                    throw std::invalid_argument(
+                        "cannot save zoned event '" + event.uid + "' with unknown TZID '"
+                        + event.time_zone + "'");
+                }
             }
         } else if (!event.time_zone.empty()) {
             throw std::invalid_argument(
@@ -1135,6 +1173,10 @@ LoadResult IcsStore::load(const std::filesystem::path& path) {
     }
     result.revision.contents_ = std::make_shared<const std::string>(snapshot.bytes);
 
+    parsed_vtimezones.clear();
+    inside_vtimezone = false;
+    vtimezone_has_errors = false;
+
     std::istringstream input(snapshot.bytes, std::ios::in | std::ios::binary);
     const auto lines = unfold(input);
     std::optional<PendingEvent> pending;
@@ -1252,8 +1294,11 @@ LoadResult IcsStore::load(const std::filesystem::path& path) {
                                 continue;
                             }
                             if (!warning.empty()) {
+                                const bool is_tzid = is_unknown_tzid_warning(warning);
                                 add_warning(result, property.line, std::move(warning));
-                                mark_unsafe_to_rewrite(result);
+                                if (!is_tzid) {
+                                    mark_unsafe_to_rewrite(result);
+                                }
                             }
                             if (std::find(event.recurrence_additions.begin(),
                                     event.recurrence_additions.end(), parsed->value)
@@ -1275,8 +1320,11 @@ LoadResult IcsStore::load(const std::filesystem::path& path) {
                                 continue;
                             }
                             if (!warning.empty()) {
+                                const bool is_tzid = is_unknown_tzid_warning(warning);
                                 add_warning(result, property.line, std::move(warning));
-                                mark_unsafe_to_rewrite(result);
+                                if (!is_tzid) {
+                                    mark_unsafe_to_rewrite(result);
+                                }
                             }
                             if (std::find(event.recurrence_exceptions.begin(),
                                     event.recurrence_exceptions.end(), parsed->value)
@@ -1298,6 +1346,39 @@ LoadResult IcsStore::load(const std::filesystem::path& path) {
         }
 
         if (!pending) {
+            // Handle embedded VTIMEZONE for safety analysis and round-trip.
+            if (name == "BEGIN" && upper(value) == "VTIMEZONE") {
+                if (inside_vtimezone) {
+                    add_warning(result, index + 1, "nested VTIMEZONE is not supported");
+                    mark_unsafe_to_rewrite(result);
+                } else {
+                    inside_vtimezone = true;
+                    vtimezone_has_errors = false;
+                    parsed_vtimezones.emplace_back();
+                }
+                continue;
+            }
+            if (name == "END" && upper(value) == "VTIMEZONE") {
+                if (!inside_vtimezone) {
+                    add_warning(result, index + 1, "END:VTIMEZONE without BEGIN:VTIMEZONE");
+                    mark_unsafe_to_rewrite(result);
+                } else {
+                    inside_vtimezone = false;
+                }
+                continue;
+            }
+            if (inside_vtimezone) {
+                auto& vt = parsed_vtimezones.back();
+                if (!vt.raw.empty()) vt.raw += "\r\n";
+                vt.raw += std::string(line);
+                if (name == "TZID" && vt.tzid.empty()) {
+                    vt.tzid = std::string(value);
+                }
+                // Sub-components and their properties are tracked but not deeply
+                // parsed; unsupported content inside VTIMEZONE is deferred.
+                continue;
+            }
+
             const std::string normalized_value = upper(value);
             const bool structural = (name == "BEGIN" && normalized_value == "VCALENDAR")
                 || (name == "END" && normalized_value == "VCALENDAR");
@@ -1366,8 +1447,11 @@ LoadResult IcsStore::load(const std::filesystem::path& path) {
                 mark_unsafe_to_rewrite(result);
             } else {
                 if (!warning.empty()) {
+                    const bool is_tzid = is_unknown_tzid_warning(warning);
                     add_warning(result, index + 1, std::move(warning));
-                    mark_unsafe_to_rewrite(result);
+                    if (!is_tzid) {
+                        mark_unsafe_to_rewrite(result);
+                    }
                 }
                 destination = *parsed;
             }
@@ -1378,6 +1462,59 @@ LoadResult IcsStore::load(const std::filesystem::path& path) {
         add_warning(result, event_start_line, "unterminated VEVENT was skipped");
         mark_unsafe_to_rewrite(result);
     }
+    if (inside_vtimezone) {
+        add_warning(result, 0, "unterminated VTIMEZONE; timezone rules may be incomplete");
+        mark_unsafe_to_rewrite(result);
+    }
+
+    // VTIMEZONE-defined TZIDs that are unknown to the system are safe to edit
+    // (we can round-trip the definition) but DST handling is approximate.
+    for (const auto& event : result.events) {
+        if (event.time_basis == TimeBasis::zoned && !event.time_zone.empty()) {
+            const std::string& tzid = event.time_zone;
+            try {
+                (void)locate_zone(tzid);
+            } catch (const std::runtime_error&) {
+                if (vtimezone_defined(tzid)) {
+                    add_warning(result, 0,
+                        "TZID '" + tzid + "' is defined by VTIMEZONE but not by the system; "
+                        "DST transitions use local time as an approximation");
+                }
+            }
+        }
+    }
+
+    // Remove "unknown TZID" warnings only for TZIDs defined in VTIMEZONE.
+    auto safe_end = std::remove_if(result.warnings.begin(), result.warnings.end(),
+        [](const std::string& w) {
+            const auto marker = w.find("unknown TZID=");
+            if (marker == std::string::npos) return false;
+            // Extract TZID from warning like "line N: unknown TZID='Custom/Test'; ..."
+            const std::size_t start = w.find('\'', marker);
+            const std::size_t end = w.find('\'', start + 1);
+            if (start == std::string::npos || end == std::string::npos) return false;
+            const std::string tzid = w.substr(start + 1, end - start - 1);
+            return vtimezone_defined(tzid);
+        });
+    result.warnings.erase(safe_end, result.warnings.end());
+
+    // If no undefined-TZID warnings remain and only VTIMEZONE-DST warnings
+    // (safe to edit) are present, mark the calendar as safe to rewrite.
+    bool has_undefined_tzid = std::any_of(result.warnings.begin(), result.warnings.end(),
+        [](const std::string& w) { return w.find("unknown TZID=") != std::string::npos; });
+    if (has_undefined_tzid) {
+        mark_unsafe_to_rewrite(result);
+    } else if (!result.warnings.empty()) {
+        bool has_unsafe_warning = std::any_of(result.warnings.begin(), result.warnings.end(),
+            [](const std::string& w) {
+                return w.find("VTIMEZONE") == std::string::npos || w.find("DST") == std::string::npos;
+            });
+        if (!has_unsafe_warning) {
+            result.safe_to_rewrite = true;
+        }
+    }
+
+    result.vtimezones = std::move(parsed_vtimezones);
     return result;
 }
 
@@ -1389,16 +1526,17 @@ std::filesystem::path IcsStore::backup_path(const std::filesystem::path& path) {
 
 FileRevision IcsStore::save(
     const std::filesystem::path& path, std::span<const Event> events,
-    std::optional<FileRevision> expected) {
+    std::optional<FileRevision> expected,
+    std::span<const VTimezoneDefinition> vtimezones) {
     if (path.empty() || path.filename().empty()) {
         throw std::invalid_argument("calendar path must name a file");
     }
     for (const Event& event : events) {
-        validate_event_for_save(event);
+        validate_event_for_save(event, vtimezones);
     }
 
     std::ostringstream serialized(std::ios::out | std::ios::binary);
-    serialize_calendar(serialized, events);
+    serialize_calendar(serialized, events, vtimezones);
     if (!serialized) {
         throw std::runtime_error("unable to serialize calendar '" + path.string() + "'");
     }
diff --git a/tests/ics_tests.cpp b/tests/ics_tests.cpp
index 261e05c..036c868 100644
--- a/tests/ics_tests.cpp
+++ b/tests/ics_tests.cpp
@@ -697,8 +697,14 @@ void test_unsupported_recurrence_and_zones_are_unsafe(const std::filesystem::pat
         "END:VTIMEZONE\r\nBEGIN:VEVENT\r\nUID:custom\r\n"
         "DTSTART:20260717T090000Z\r\nDTEND:20260717T100000Z\r\n"
         "END:VEVENT\r\nEND:VCALENDAR\r\n");
-    require(!nocal::storage::IcsStore::load(custom_zone).safe_to_rewrite,
-        "VTIMEZONE definition was considered rewrite-safe");
+    // VTIMEZONE is now parsed and preserved for round-trip, so the calendar
+    // is safe to rewrite even though the custom TZID is unused by the event.
+    const auto vtz_result = nocal::storage::IcsStore::load(custom_zone);
+    require(vtz_result.safe_to_rewrite,
+        "VTIMEZONE definition should be safe when round-trippable");
+    require(vtz_result.vtimezones.size() == 1 &&
+            vtz_result.vtimezones[0].tzid == "Custom/Test",
+        "VTIMEZONE should be captured even when no event references it");
 
     struct InvalidRdate {
         std::string start;
@@ -813,6 +819,131 @@ void test_save_validation_preserves_existing_bytes(const std::filesystem::path&
     require_no_temporary_files(path);
 }
 
+void test_vtimezone_custom_tzid_safe_and_round_trips(const std::filesystem::path& root) {
+    const auto path = root / "vtimezone-custom.ics";
+    write_fixture(path,
+        "BEGIN:VCALENDAR\r\n"
+        "VERSION:2.0\r\n"
+        "BEGIN:VTIMEZONE\r\n"
+        "TZID:Custom/Example\r\n"
+        "BEGIN:STANDARD\r\n"
+        "TZOFFSETFROM:+0100\r\n"
+        "TZOFFSETTO:+0000\r\n"
+        "DTSTART:19701025T030000\r\n"
+        "RRULE:FREQ=YEARLY;BYDAY=-1SU;BYMONTH=10\r\n"
+        "END:STANDARD\r\n"
+        "BEGIN:DAYLIGHT\r\n"
+        "TZOFFSETFROM:+0000\r\n"
+        "TZOFFSETTO:+0100\r\n"
+        "DTSTART:19700329T020000\r\n"
+        "RRULE:FREQ=YEARLY;BYDAY=-1SU;BYMONTH=3\r\n"
+        "END:DAYLIGHT\r\n"
+        "END:VTIMEZONE\r\n"
+        "BEGIN:VEVENT\r\n"
+        "UID:custom-tz\r\n"
+        "DTSTART;TZID=Custom/Example:20260717T100000\r\n"
+        "DTEND;TZID=Custom/Example:20260717T110000\r\n"
+        "SUMMARY:Custom zone event\r\n"
+        "END:VEVENT\r\n"
+        "END:VCALENDAR\r\n");
+
+    const auto loaded = nocal::storage::IcsStore::load(path);
+    require(loaded.safe_to_rewrite,
+        "VTIMEZONE-defined TZID should make the calendar safe to rewrite");
+    require(loaded.events.size() == 1, "VTIMEZONE calendar should contain the event");
+    require(loaded.events[0].time_zone == "Custom/Example",
+        "VTIMEZONE-defined TZID should be retained on the event");
+    require(loaded.events[0].time_basis == nocal::TimeBasis::zoned,
+        "zoned time basis should be preserved");
+    require(loaded.vtimezones.size() == 1,
+        "VTIMEZONE definition should be captured");
+    require(loaded.vtimezones[0].tzid == "Custom/Example",
+        "VTIMEZONE TZID should match");
+    require(!loaded.vtimezones[0].raw.empty(),
+        "VTIMEZONE raw content should be captured");
+
+    // Verify a VTIMEZONE-DST warning was emitted for unknown TZID.
+    bool has_dst_warning = false;
+    for (const auto& w : loaded.warnings) {
+        if (w.find("DST") != std::string::npos) {
+            has_dst_warning = true;
+            break;
+        }
+    }
+    require(has_dst_warning,
+        "should warn that DST handling is approximate for custom VTIMEZONE");
+
+    // Round-trip: save with VTIMEZONE preserved, then reload.
+    const auto saved_revision = nocal::storage::IcsStore::save(
+        path, loaded.events, loaded.revision, loaded.vtimezones);
+    require(saved_revision.existed(), "save should return an existing revision");
+
+    const std::string bytes = read_fixture(path);
+    require(bytes.find("BEGIN:VTIMEZONE") != std::string::npos,
+        "VTIMEZONE block should be serialized on save");
+    require(bytes.find("TZID:Custom/Example") != std::string::npos,
+        "VTIMEZONE TZID should appear in serialized output");
+    require(bytes.find("BEGIN:STANDARD") != std::string::npos,
+        "VTIMEZONE sub-components should be preserved");
+
+    const auto reloaded = nocal::storage::IcsStore::load(path);
+    require(reloaded.safe_to_rewrite, "round-tripped VTIMEZONE calendar should be safe");
+    require(reloaded.events.size() == 1, "round-trip event count should match");
+    require(reloaded.events[0].time_zone == "Custom/Example",
+        "round-trip TZID should be preserved");
+    require(reloaded.vtimezones.size() == 1,
+        "round-trip VTIMEZONE count should match");
+    require(reloaded.vtimezones[0].tzid == "Custom/Example",
+        "round-trip VTIMEZONE TZID should match");
+    require_no_temporary_files(path);
+}
+
+void test_vtimezone_multiple_tzids(const std::filesystem::path& root) {
+    const auto path = root / "vtimezone-multiple.ics";
+    write_fixture(path,
+        "BEGIN:VCALENDAR\r\n"
+        "VERSION:2.0\r\n"
+        "BEGIN:VTIMEZONE\r\nTZID:Custom/A\r\nEND:VTIMEZONE\r\n"
+        "BEGIN:VTIMEZONE\r\nTZID:Custom/B\r\nEND:VTIMEZONE\r\n"
+        "BEGIN:VEVENT\r\n"
+        "UID:evt-a\r\n"
+        "DTSTART;TZID=Custom/A:20260717T100000\r\n"
+        "DTEND;TZID=Custom/A:20260717T110000\r\n"
+        "SUMMARY:Event A\r\n"
+        "END:VEVENT\r\n"
+        "BEGIN:VEVENT\r\n"
+        "UID:evt-b\r\n"
+        "DTSTART;TZID=Custom/B:20260718T100000\r\n"
+        "DTEND;TZID=Custom/B:20260718T110000\r\n"
+        "SUMMARY:Event B\r\n"
+        "END:VEVENT\r\n"
+        "END:VCALENDAR\r\n");
+
+    const auto loaded = nocal::storage::IcsStore::load(path);
+    require(loaded.safe_to_rewrite,
+        "multiple VTIMEZONE-defined TZIDs should be safe");
+    require(loaded.events.size() == 2, "should parse all events");
+    require(loaded.vtimezones.size() == 2, "should capture both VTIMEZONE definitions");
+}
+
+void test_vtimezone_undefined_tzid_still_unsafe(const std::filesystem::path& root) {
+    const auto path = root / "vtimezone-undefined.ics";
+    // Event uses Custom/Unknown which is NOT defined in any VTIMEZONE.
+    write_fixture(path,
+        "BEGIN:VCALENDAR\r\nVERSION:2.0\r\n"
+        "BEGIN:VTIMEZONE\r\nTZID:Custom/Defined\r\nEND:VTIMEZONE\r\n"
+        "BEGIN:VEVENT\r\nUID:undef\r\n"
+        "DTSTART;TZID=Custom/Unknown:20260717T100000\r\n"
+        "DTEND;TZID=Custom/Unknown:20260717T110000\r\n"
+        "SUMMARY:Undefined TZID\r\nEND:VEVENT\r\n"
+        "END:VCALENDAR\r\n");
+
+    const auto loaded = nocal::storage::IcsStore::load(path);
+    require(!loaded.safe_to_rewrite,
+        "undefined TZID should still be unsafe even when other VTIMEZONEs exist");
+    require(loaded.events.size() == 1, "event should still be browseable");
+}
+
 } // namespace
 
 int main() {
@@ -843,6 +974,9 @@ int main() {
         test_supported_rdates(root);
         test_unsupported_recurrence_and_zones_are_unsafe(root);
         test_save_validation_preserves_existing_bytes(root);
+        test_vtimezone_custom_tzid_safe_and_round_trips(root);
+        test_vtimezone_multiple_tzids(root);
+        test_vtimezone_undefined_tzid_still_unsafe(root);
         std::filesystem::remove_all(root, ignored);
         std::cout << "ics_tests: ok\n";
         return 0;