diff --git a/internal/caldav/backend.go b/internal/caldav/backend.go
index e2f4120..5326d81 100644
--- a/internal/caldav/backend.go
+++ b/internal/caldav/backend.go
@@ -20,6 +20,7 @@ import (
 	"github.com/yourusername/caldav-server/internal/auth"
 	"github.com/yourusername/caldav-server/internal/config"
 	"github.com/yourusername/caldav-server/internal/db"
+	"github.com/yourusername/caldav-server/internal/icalfix"
 	"github.com/yourusername/caldav-server/internal/icssub"
 	"github.com/yourusername/caldav-server/internal/store"
 )
@@ -383,7 +384,13 @@ func (b *Backend) calendarMeta(owner, realName, localName string) caldav.Calenda
 }
 
 func (b *Backend) decodeObject(objPath string, data []byte) (*caldav.CalendarObject, error) {
-	cal, err := ical.NewDecoder(strings.NewReader(string(data))).Decode()
+	// Some clients (Outlook/Exchange, some Thunderbird/Lightning setups)
+	// write Windows timezone names into TZID instead of IANA ones, which
+	// go-ical can't resolve. A recurring event with such a TZID decodes
+	// fine here but fails later, as a 500, the moment a CalDAV client
+	// issues a time-range query that expands its recurrence — so fix it
+	// up before decoding rather than only in QueryCalendarObjects.
+	cal, err := ical.NewDecoder(bytes.NewReader(icalfix.NormalizeTimeZones(data))).Decode()
 	if err != nil {
 		return nil, fmt.Errorf("decoding ical: %w", err)
 	}
diff --git a/internal/icalfix/icalfix.go b/internal/icalfix/icalfix.go
new file mode 100644
index 0000000..1920e2e
--- /dev/null
+++ b/internal/icalfix/icalfix.go
@@ -0,0 +1,201 @@
+// Package icalfix normalizes non-standard timezone identifiers in raw
+// iCalendar data before it's handed to go-ical's decoder.
+//
+// go-ical resolves a DTSTART/DTEND's TZID parameter (and, for recurring
+// events, the RRULE expansion's start time) via a direct
+// time.LoadLocation(tzid) call — see
+// github.com/emersion/go-ical@.../ical.go's Prop.DateTime and
+// components.go's Component.RecurrenceSet. That only understands IANA
+// zone names ("Europe/Berlin"), but Windows/Outlook (and anything built
+// on the Windows timezone database, e.g. Exchange, Thunderbird's Lightning
+// on some setups) commonly emits Windows zone names instead
+// ("W. Europe Standard Time"), which time.LoadLocation doesn't recognize.
+// A recurring event stored with such a TZID decodes fine (go-ical doesn't
+// eagerly parse RRULE dates), but fails later — as a 500 — the moment a
+// CalDAV client issues a time-range REPORT that has to expand its
+// recurrence, since that's when RecurrenceSet actually calls
+// time.LoadLocation.
+package icalfix
+
+import "regexp"
+
+// tzidParamRe matches a "TZID=<name>" (optionally quoted) parameter value
+// as it appears attached to any property (e.g. "DTSTART;TZID=W. Europe
+// Standard Time:..."). The name runs until the next ':' or ';' delimiter
+// or line break, so it captures the parameter's full value even though it
+// contains spaces and periods.
+var tzidParamRe = regexp.MustCompile(`TZID=("?)([^";:\r\n]+)("?)`)
+
+// tzidLineRe matches a standalone "TZID:<name>" property line, as found
+// inside a VTIMEZONE component.
+var tzidLineRe = regexp.MustCompile(`(?m)^TZID:([^\r\n]+)$`)
+
+// NormalizeTimeZones rewrites any recognized Windows timezone identifier
+// in data to its IANA equivalent, leaving everything else (including
+// unrecognized TZIDs) untouched. It's safe to call on data that has no
+// Windows TZIDs at all — such data passes through unchanged.
+func NormalizeTimeZones(data []byte) []byte {
+	data = tzidParamRe.ReplaceAllFunc(data, func(m []byte) []byte {
+		sub := tzidParamRe.FindSubmatch(m)
+		name := string(sub[2])
+		iana, ok := windowsToIANA[name]
+		if !ok {
+			return m
+		}
+		return []byte("TZID=" + string(sub[1]) + iana + string(sub[3]))
+	})
+	data = tzidLineRe.ReplaceAllFunc(data, func(m []byte) []byte {
+		sub := tzidLineRe.FindSubmatch(m)
+		name := string(sub[1])
+		iana, ok := windowsToIANA[name]
+		if !ok {
+			return m
+		}
+		return []byte("TZID:" + iana)
+	})
+	return data
+}
+
+// windowsToIANA maps common Windows timezone identifiers (as found in the
+// CLDR windowsZones.xml "001"/default territory mapping) to an IANA zone
+// name recognized by Go's tzdata. It's not exhaustive, but covers the
+// zones seen in practice from Outlook/Exchange-generated ICS data.
+var windowsToIANA = map[string]string{
+	"Dateline Standard Time":          "Etc/GMT+12",
+	"UTC-11":                          "Etc/GMT+11",
+	"Aleutian Standard Time":          "America/Adak",
+	"Hawaiian Standard Time":          "Pacific/Honolulu",
+	"Marquesas Standard Time":         "Pacific/Marquesas",
+	"Alaskan Standard Time":           "America/Anchorage",
+	"UTC-09":                          "Etc/GMT+9",
+	"Pacific Standard Time (Mexico)":  "America/Tijuana",
+	"UTC-08":                          "Etc/GMT+8",
+	"Pacific Standard Time":           "America/Los_Angeles",
+	"US Mountain Standard Time":       "America/Phoenix",
+	"Mountain Standard Time (Mexico)": "America/Chihuahua",
+	"Mountain Standard Time":          "America/Denver",
+	"Central America Standard Time":   "America/Guatemala",
+	"Central Standard Time":           "America/Chicago",
+	"Central Standard Time (Mexico)":  "America/Mexico_City",
+	"Canada Central Standard Time":    "America/Regina",
+	"SA Pacific Standard Time":        "America/Bogota",
+	"Eastern Standard Time (Mexico)":  "America/Cancun",
+	"Eastern Standard Time":           "America/New_York",
+	"Haiti Standard Time":             "America/Port-au-Prince",
+	"Cuba Standard Time":              "America/Havana",
+	"US Eastern Standard Time":        "America/Indianapolis",
+	"Turks And Caicos Standard Time":  "America/Grand_Turk",
+	"Paraguay Standard Time":          "America/Asuncion",
+	"Atlantic Standard Time":          "America/Halifax",
+	"Venezuela Standard Time":         "America/Caracas",
+	"Central Brazilian Standard Time": "America/Cuiaba",
+	"SA Western Standard Time":        "America/La_Paz",
+	"Pacific SA Standard Time":        "America/Santiago",
+	"Newfoundland Standard Time":      "America/St_Johns",
+	"Tocantins Standard Time":         "America/Araguaina",
+	"E. South America Standard Time":  "America/Sao_Paulo",
+	"SA Eastern Standard Time":        "America/Cayenne",
+	"Argentina Standard Time":         "America/Buenos_Aires",
+	"Greenland Standard Time":         "America/Godthab",
+	"Montevideo Standard Time":        "America/Montevideo",
+	"Magallanes Standard Time":        "America/Punta_Arenas",
+	"Saint Pierre Standard Time":      "America/Miquelon",
+	"Bahia Standard Time":             "America/Bahia",
+	"UTC-02":                          "Etc/GMT+2",
+	"Azores Standard Time":            "Atlantic/Azores",
+	"Cape Verde Standard Time":        "Atlantic/Cape_Verde",
+	"UTC":                             "Etc/UTC",
+	"GMT Standard Time":               "Europe/London",
+	"Greenwich Standard Time":         "Atlantic/Reykjavik",
+	"Sao Tome Standard Time":          "Africa/Sao_Tome",
+	"Morocco Standard Time":           "Africa/Casablanca",
+	"W. Europe Standard Time":         "Europe/Berlin",
+	"Central Europe Standard Time":    "Europe/Budapest",
+	"Romance Standard Time":           "Europe/Paris",
+	"Central European Standard Time":  "Europe/Warsaw",
+	"W. Central Africa Standard Time": "Africa/Lagos",
+	"Jordan Standard Time":            "Asia/Amman",
+	"GTB Standard Time":               "Europe/Bucharest",
+	"Middle East Standard Time":       "Asia/Beirut",
+	"Egypt Standard Time":             "Africa/Cairo",
+	"E. Europe Standard Time":         "Europe/Chisinau",
+	"Syria Standard Time":             "Asia/Damascus",
+	"West Bank Standard Time":         "Asia/Hebron",
+	"South Africa Standard Time":      "Africa/Johannesburg",
+	"FLE Standard Time":               "Europe/Kiev",
+	"Israel Standard Time":            "Asia/Jerusalem",
+	"Kaliningrad Standard Time":       "Europe/Kaliningrad",
+	"Sudan Standard Time":             "Africa/Khartoum",
+	"Libya Standard Time":             "Africa/Tripoli",
+	"Namibia Standard Time":           "Africa/Windhoek",
+	"Arabic Standard Time":            "Asia/Baghdad",
+	"Turkey Standard Time":            "Europe/Istanbul",
+	"Arab Standard Time":              "Asia/Riyadh",
+	"Belarus Standard Time":           "Europe/Minsk",
+	"Russian Standard Time":           "Europe/Moscow",
+	"E. Africa Standard Time":         "Africa/Nairobi",
+	"Volgograd Standard Time":         "Europe/Volgograd",
+	"Iran Standard Time":              "Asia/Tehran",
+	"Arabian Standard Time":           "Asia/Dubai",
+	"Astrakhan Standard Time":         "Europe/Astrakhan",
+	"Azerbaijan Standard Time":        "Asia/Baku",
+	"Russia Time Zone 3":              "Europe/Samara",
+	"Mauritius Standard Time":         "Indian/Mauritius",
+	"Saratov Standard Time":           "Europe/Saratov",
+	"Georgian Standard Time":          "Asia/Tbilisi",
+	"Caucasus Standard Time":          "Asia/Yerevan",
+	"Afghanistan Standard Time":       "Asia/Kabul",
+	"West Asia Standard Time":         "Asia/Tashkent",
+	"Ekaterinburg Standard Time":      "Asia/Yekaterinburg",
+	"Pakistan Standard Time":          "Asia/Karachi",
+	"Qyzylorda Standard Time":         "Asia/Qyzylorda",
+	"India Standard Time":             "Asia/Calcutta",
+	"Sri Lanka Standard Time":         "Asia/Colombo",
+	"Nepal Standard Time":             "Asia/Katmandu",
+	"Central Asia Standard Time":      "Asia/Almaty",
+	"Bangladesh Standard Time":        "Asia/Dhaka",
+	"Omsk Standard Time":              "Asia/Omsk",
+	"Myanmar Standard Time":           "Asia/Rangoon",
+	"SE Asia Standard Time":           "Asia/Bangkok",
+	"Altai Standard Time":             "Asia/Barnaul",
+	"W. Mongolia Standard Time":       "Asia/Hovd",
+	"North Asia Standard Time":        "Asia/Krasnoyarsk",
+	"N. Central Asia Standard Time":   "Asia/Novosibirsk",
+	"Tomsk Standard Time":             "Asia/Tomsk",
+	"China Standard Time":             "Asia/Shanghai",
+	"North Asia East Standard Time":   "Asia/Irkutsk",
+	"Singapore Standard Time":         "Asia/Singapore",
+	"W. Australia Standard Time":      "Australia/Perth",
+	"Taipei Standard Time":            "Asia/Taipei",
+	"Ulaanbaatar Standard Time":       "Asia/Ulaanbaatar",
+	"Aus Central W. Standard Time":    "Australia/Eucla",
+	"Transbaikal Standard Time":       "Asia/Chita",
+	"Tokyo Standard Time":             "Asia/Tokyo",
+	"North Korea Standard Time":       "Asia/Pyongyang",
+	"Korea Standard Time":             "Asia/Seoul",
+	"Yakutsk Standard Time":           "Asia/Yakutsk",
+	"Cen. Australia Standard Time":    "Australia/Adelaide",
+	"AUS Central Standard Time":       "Australia/Darwin",
+	"E. Australia Standard Time":      "Australia/Brisbane",
+	"AUS Eastern Standard Time":       "Australia/Sydney",
+	"West Pacific Standard Time":      "Pacific/Port_Moresby",
+	"Tasmania Standard Time":          "Australia/Hobart",
+	"Vladivostok Standard Time":       "Asia/Vladivostok",
+	"Lord Howe Standard Time":         "Australia/Lord_Howe",
+	"Bougainville Standard Time":      "Pacific/Bougainville",
+	"Russia Time Zone 10":             "Asia/Srednekolymsk",
+	"Magadan Standard Time":           "Asia/Magadan",
+	"Norfolk Standard Time":           "Pacific/Norfolk",
+	"Sakhalin Standard Time":          "Asia/Sakhalin",
+	"Central Pacific Standard Time":   "Pacific/Guadalcanal",
+	"Russia Time Zone 11":             "Asia/Kamchatka",
+	"New Zealand Standard Time":       "Pacific/Auckland",
+	"UTC+12":                          "Etc/GMT-12",
+	"Fiji Standard Time":              "Pacific/Fiji",
+	"Kamchatka Standard Time":         "Asia/Kamchatka",
+	"Chatham Islands Standard Time":   "Pacific/Chatham",
+	"UTC+13":                          "Etc/GMT-13",
+	"Tonga Standard Time":             "Pacific/Tongatapu",
+	"Samoa Standard Time":             "Pacific/Apia",
+	"Line Islands Standard Time":      "Pacific/Kiritimati",
+}
diff --git a/internal/icalfix/icalfix_test.go b/internal/icalfix/icalfix_test.go
new file mode 100644
index 0000000..abe120a
--- /dev/null
+++ b/internal/icalfix/icalfix_test.go
@@ -0,0 +1,71 @@
+package icalfix
+
+import (
+	"bytes"
+	"testing"
+
+	ical "github.com/emersion/go-ical"
+)
+
+// TestNormalizeTimeZonesFixesRecurringEvent reproduces the bug reported by
+// a DAVx5 client: a recurring VEVENT with a Windows-style TZID
+// ("W. Europe Standard Time") decodes fine, but expanding its recurrence
+// via RecurrenceSet (as go-webdav's caldav.Filter does for a time-range
+// REPORT) fails with "unknown time zone" because go-ical resolves TZID
+// via a plain time.LoadLocation call. Normalizing the TZID to its IANA
+// equivalent before decoding must make that expansion succeed.
+func TestNormalizeTimeZonesFixesRecurringEvent(t *testing.T) {
+	const raw = "BEGIN:VCALENDAR\r\n" +
+		"VERSION:2.0\r\n" +
+		"PRODID:-//Test//Test//EN\r\n" +
+		"BEGIN:VTIMEZONE\r\n" +
+		"TZID:W. Europe Standard Time\r\n" +
+		"END:VTIMEZONE\r\n" +
+		"BEGIN:VEVENT\r\n" +
+		"UID:recurring@example.com\r\n" +
+		"DTSTAMP:20240101T000000Z\r\n" +
+		"DTSTART;TZID=W. Europe Standard Time:20260522T220000\r\n" +
+		"DTEND;TZID=W. Europe Standard Time:20260522T230000\r\n" +
+		"RRULE:FREQ=WEEKLY;COUNT=5\r\n" +
+		"SUMMARY:Weekly sync\r\n" +
+		"END:VEVENT\r\n" +
+		"END:VCALENDAR\r\n"
+
+	normalized := NormalizeTimeZones([]byte(raw))
+
+	cal, err := ical.NewDecoder(bytes.NewReader(normalized)).Decode()
+	if err != nil {
+		t.Fatalf("decoding normalized calendar: %v", err)
+	}
+	events := cal.Events()
+	if len(events) != 1 {
+		t.Fatalf("expected 1 event, got %d", len(events))
+	}
+
+	if _, err := events[0].RecurrenceSet(nil); err != nil {
+		t.Fatalf("expanding recurrence after normalization: %v", err)
+	}
+
+	// The un-normalized original must reproduce the reported failure, so
+	// this test actually demonstrates the fix rather than trivially
+	// passing regardless.
+	origCal, err := ical.NewDecoder(bytes.NewReader([]byte(raw))).Decode()
+	if err != nil {
+		t.Fatalf("decoding original calendar: %v", err)
+	}
+	if _, err := origCal.Events()[0].RecurrenceSet(nil); err == nil {
+		t.Fatalf("expected RecurrenceSet to fail on un-normalized Windows TZID, it didn't")
+	}
+}
+
+func TestNormalizeTimeZonesLeavesUnknownAndIANAZonesAlone(t *testing.T) {
+	const raw = "BEGIN:VCALENDAR\r\n" +
+		"DTSTART;TZID=Europe/Berlin:20260522T220000\r\n" +
+		"DTEND;TZID=Some/Unknown-Zone:20260522T230000\r\n" +
+		"END:VCALENDAR\r\n"
+
+	got := string(NormalizeTimeZones([]byte(raw)))
+	if got != raw {
+		t.Fatalf("expected data to be unchanged, got:\n%s", got)
+	}
+}
diff --git a/internal/icssub/icssub.go b/internal/icssub/icssub.go
index ae91293..5eb7610 100644
--- a/internal/icssub/icssub.go
+++ b/internal/icssub/icssub.go
@@ -5,6 +5,7 @@
 package icssub
 
 import (
+	"bytes"
 	"context"
 	"fmt"
 	"io"
@@ -14,6 +15,8 @@ import (
 	"time"
 
 	ical "github.com/emersion/go-ical"
+
+	"github.com/yourusername/caldav-server/internal/icalfix"
 )
 
 // DefaultTTL is how long a fetched calendar is cached before being
@@ -103,7 +106,14 @@ func (c *Cache) fetch(rawURL string) (*ical.Calendar, error) {
 		return nil, fmt.Errorf("fetching calendar: unexpected status %s", resp.Status)
 	}
 
-	cal, err := ical.NewDecoder(io.LimitReader(resp.Body, maxBodySize)).Decode()
+	body, err := io.ReadAll(io.LimitReader(resp.Body, maxBodySize))
+	if err != nil {
+		return nil, fmt.Errorf("reading calendar: %w", err)
+	}
+	// Some remote feeds (Outlook/Exchange-backed ones especially) use
+	// Windows timezone names instead of IANA ones, which go-ical can't
+	// resolve — fix those up before decoding (see internal/icalfix).
+	cal, err := ical.NewDecoder(bytes.NewReader(icalfix.NormalizeTimeZones(body))).Decode()
 	if err != nil {
 		return nil, fmt.Errorf("parsing calendar: %w", err)
 	}
diff --git a/internal/web/calendar.go b/internal/web/calendar.go
index 4b387f3..a5f90f6 100644
--- a/internal/web/calendar.go
+++ b/internal/web/calendar.go
@@ -18,6 +18,7 @@ import (
 	ical "github.com/emersion/go-ical"
 	"github.com/yourusername/caldav-server/internal/birthdays"
 	"github.com/yourusername/caldav-server/internal/db"
+	"github.com/yourusername/caldav-server/internal/icalfix"
 	"github.com/yourusername/caldav-server/internal/web/templates"
 )
 
@@ -675,7 +676,7 @@ func (s *Server) saveEventFromForm(w http.ResponseWriter, r *http.Request, owner
 			http.NotFound(w, r)
 			return
 		}
-		existing, err := ical.NewDecoder(strings.NewReader(string(data))).Decode()
+		existing, err := ical.NewDecoder(bytes.NewReader(icalfix.NormalizeTimeZones(data))).Decode()
 		if err != nil {
 			s.logger.Error("decoding existing event", "error", err)
 			http.Error(w, "internal error", http.StatusInternalServerError)
@@ -791,7 +792,7 @@ func (s *Server) saveEvent(owner, cal, id string, calendar *ical.Calendar) error
 // EventFormData shape used both for pre-filling the edit form and for
 // placing the event on the month grid.
 func eventFormFromICS(id string, data []byte) (templates.EventFormData, error) {
-	calendar, err := ical.NewDecoder(strings.NewReader(string(data))).Decode()
+	calendar, err := ical.NewDecoder(bytes.NewReader(icalfix.NormalizeTimeZones(data))).Decode()
 	if err != nil {
 		return templates.EventFormData{}, err
 	}
@@ -974,7 +975,7 @@ func (s *Server) handleCalendarImport(w http.ResponseWriter, r *http.Request) {
 	}
 
 	imported := 0
-	dec := ical.NewDecoder(bytes.NewReader(body))
+	dec := ical.NewDecoder(bytes.NewReader(icalfix.NormalizeTimeZones(body)))
 	for {
 		srcCal, err := dec.Decode()
 		if errors.Is(err, io.EOF) {