fix(macos): Sunday week start and UTC timezone labels like web

apps/calendar-macos/CompassData/Sources/CompassData/NativeCalendarRootModel.swift

@@ -174,7 +174,7 @@ public final class NativeCalendarRootModel {
 
         let anchor: Date = {
             let today = demoPresentation?.referenceNow ?? now()
-            return EffectiveTimeZone.calendar.dateInterval(of: .weekOfYear, for: today)?.start ?? today
+            return CalendarWindowMath.sundayWeekViewStart(containing: today)
         }()
         viewStore = ViewStore(
             view: .week,
@@ -372,9 +372,7 @@ public final class NativeCalendarRootModel {
         contentTrackWidth = CGFloat(ParityCaptureGeometry.contentTrackWidth)
         viewStore.setVisibleDayCount(ParityCaptureGeometry.visibleDayCount)
         let today = referenceNow
-        let calendar = EffectiveTimeZone.calendar
-        let weekStart = calendar.dateInterval(of: .weekOfYear, for: today)?.start ?? today
-        viewStore.setAnchorDate(weekStart)
+        viewStore.setAnchorDate(CalendarWindowMath.sundayWeekViewStart(containing: today))
         monthPickerMonth = today
         pendingScroll = .scrollToNowOrPulse
         rebuildPresentation()
@@ -922,8 +920,7 @@ public final class NativeCalendarRootModel {
         if viewStore.visibleDayCount == 1 {
             viewStore.setAnchorDate(today)
         } else {
-            let weekStart = calendar.dateInterval(of: .weekOfYear, for: today)?.start ?? today
-            viewStore.setAnchorDate(weekStart)
+            viewStore.setAnchorDate(CalendarWindowMath.sundayWeekViewStart(containing: today))
         }
         monthPickerMonth = today
         rebuildPresentation()

apps/calendar-macos/CompassKit/Sources/CompassKit/Calendar/CalendarWindowMath.swift

@@ -3,6 +3,17 @@ import Foundation
 public enum CalendarWindowMath {
     public static let weekDayCount = 7
 
+    /// Web Day.js `startOf("week")`: Sunday 00:00 in the effective timezone.
+    public static func sundayWeekViewStart(containing date: Date) -> Date {
+        var calendar = EffectiveTimeZone.calendar
+        calendar.firstWeekday = 1
+        calendar.minimumDaysInFirstWeek = 1
+        let startOfDay = calendar.startOfDay(for: date)
+        let weekday = calendar.component(.weekday, from: startOfDay)
+        let daysFromSunday = (weekday - calendar.firstWeekday + 7) % 7
+        return calendar.date(byAdding: .day, value: -daysFromSunday, to: startOfDay) ?? startOfDay
+    }
+
     /// Seven local days from the anchor's start of day; `end` is exclusive next midnight.
     public static func weekEventsQueryWindow(anchor: Date) -> DateRange {
         let calendar = EffectiveTimeZone.calendar

apps/calendar-macos/CompassKit/Sources/CompassKit/Timezone/TimeZoneFormatting.swift

@@ -5,17 +5,54 @@ public enum TimeZoneFormatting {
         TimeZone(identifier: identifier) != nil
     }
 
+    /// Matches web `formatTimeZoneAbbreviation` (Intl short, offset fallback).
     public static func abbreviation(for timeZoneId: String, at date: Date = Date()) -> String {
         guard let zone = TimeZone(identifier: timeZoneId) else { return timeZoneId }
-        let formatter = DateFormatter()
-        formatter.timeZone = zone
-        formatter.dateFormat = "z"
-        return formatter.string(from: date)
+        if timeZoneId == "UTC" {
+            return "UTC"
+        }
+
+        let short = timeZoneNamePart(timeZone: zone, at: date, style: .short)
+        if let short, !looksLikeIanaId(short) {
+            return short
+        }
+
+        if let offset = timeZoneNamePart(timeZone: zone, at: date, style: .shortOffset) {
+            return offset
+        }
+        return short ?? timeZoneId
     }
 
     public static func cityName(for timeZoneId: String) -> String {
         let parts = timeZoneId.split(separator: "/")
         guard let city = parts.last else { return timeZoneId }
         return city.replacingOccurrences(of: "_", with: " ")
     }
+
+    private enum NameStyle {
+        case short
+        case shortOffset
+    }
+
+    private static func timeZoneNamePart(
+        timeZone: TimeZone,
+        at date: Date,
+        style: NameStyle
+    ) -> String? {
+        let formatter = DateFormatter()
+        formatter.locale = Locale(identifier: "en_US")
+        formatter.timeZone = timeZone
+        switch style {
+        case .short:
+            formatter.dateFormat = "zzz"
+        case .shortOffset:
+            formatter.dateFormat = "ZZZZZ"
+        }
+        let value = formatter.string(from: date)
+        return value.isEmpty ? nil : value
+    }
+
+    private static func looksLikeIanaId(_ value: String) -> Bool {
+        value.contains("/")
+    }
 }

apps/calendar-macos/CompassKitTests/CalendarWindowMathTests.swift

@@ -25,6 +25,13 @@ final class CalendarWindowMathTests: XCTestCase {
         )
     }
 
+    func testSundayWeekViewStartMatchesWeb() throws {
+        let reference = try XCTUnwrap(
+            CompassDateParsing.parseInEffectiveTimeZone("2026-06-10T15:00:00.000Z"))
+        let weekStart = CalendarWindowMath.sundayWeekViewStart(containing: reference)
+        XCTAssertEqual(CompassDateParsing.formatCalendarDay(weekStart), "2026-06-07")
+    }
+
     func testDayEventQueryRangeUsesExclusiveEnd() throws {
         let anchor = try XCTUnwrap(
             CompassDateParsing.calendarDateInEffectiveTimeZone("2026-06-28")

apps/calendar-macos/CompassKitTests/ParityCaptureGeometryTests.swift

@@ -7,4 +7,5 @@ final class ParityCaptureGeometryTests: XCTestCase {
         XCTAssertEqual(ParityCaptureGeometry.contentTrackWidth, 871, accuracy: 0.1)
         XCTAssertEqual(ParityCaptureGeometry.visibleDayCount, 5)
     }
+
 }

apps/calendar-macos/CompassKitTests/TimeZoneFormattingTests.swift

@@ -0,0 +1,18 @@
+import CompassKit
+import XCTest
+
+final class TimeZoneFormattingTests: XCTestCase {
+    func testUTCLabelMatchesWeb() throws {
+        let at = try XCTUnwrap(
+            CompassDateParsing.parseInEffectiveTimeZone("2026-07-15T12:00:00.000Z"))
+        XCTAssertEqual(TimeZoneFormatting.abbreviation(for: "UTC", at: at), "UTC")
+    }
+
+    func testNamedZoneUsesShortAbbreviation() throws {
+        let summer = try XCTUnwrap(
+            CompassDateParsing.parseInEffectiveTimeZone("2026-07-15T18:00:00.000Z"))
+        XCTAssertEqual(
+            TimeZoneFormatting.abbreviation(for: "America/Denver", at: summer),
+            "MDT")
+    }
+}