diff --git a/Sources/CalendarrCore/Complications/ComplicationTimeline.swift b/Sources/CalendarrCore/Complications/ComplicationTimeline.swift
new file mode 100644
index 0000000..06ae07f
--- /dev/null
+++ b/Sources/CalendarrCore/Complications/ComplicationTimeline.swift
@@ -0,0 +1,47 @@
+import Foundation
+
+/// When a complication needs to be redrawn.
+///
+/// A watch-face complication has a small refresh budget, so the timeline is
+/// pre-computed from data the watch already holds instead of being refreshed on
+/// a schedule: an entry on every event boundary means the complication changes
+/// in the exact minute the next appointment does, without a network call.
+public enum ComplicationTimeline {
+    /// Dates at which a complication should re-render.
+    ///
+    /// Always begins with `now` so the first entry renders immediately. Event
+    /// starts and ends within `horizon` become entries; hourly marks fill the
+    /// rest so a quiet calendar still advances its date and countdown. The
+    /// result is sorted, deduplicated to the second, and capped at
+    /// `maxEntries`, keeping the earliest.
+    public static func entryDates(from now: Date,
+                                  in snapshot: CalendarrSnapshot,
+                                  horizon: TimeInterval = 24 * 3600,
+                                  maxEntries: Int = 100) -> [Date] {
+        let end = now.addingTimeInterval(horizon)
+        var seen: Set<Int> = []
+        var dates: [Date] = []
+
+        func add(_ date: Date) {
+            guard date >= now, date <= end else { return }
+            // Deduplicate to whole seconds: two events that meet exactly would
+            // otherwise produce two entries a float apart.
+            let key = Int(date.timeIntervalSince1970.rounded())
+            guard seen.insert(key).inserted else { return }
+            dates.append(date)
+        }
+
+        add(now)
+        for event in snapshot.events {
+            add(event.start)
+            add(event.end)
+        }
+        var hourly = now.addingTimeInterval(3600)
+        while hourly <= end {
+            add(hourly)
+            hourly = hourly.addingTimeInterval(3600)
+        }
+
+        return Array(dates.sorted().prefix(maxEntries))
+    }
+}
diff --git a/Tests/CalendarrCoreTests/ComplicationTimelineTests.swift b/Tests/CalendarrCoreTests/ComplicationTimelineTests.swift
new file mode 100644
index 0000000..bfe1594
--- /dev/null
+++ b/Tests/CalendarrCoreTests/ComplicationTimelineTests.swift
@@ -0,0 +1,89 @@
+import Foundation
+import Testing
+@testable import CalendarrCore
+
+struct ComplicationTimelineTests {
+
+    private static let now = Date(timeIntervalSince1970: 1_789_000_000)
+
+    private static func event(_ id: String, startOffset: TimeInterval,
+                              duration: TimeInterval) -> SnapshotEvent {
+        SnapshotEvent(id: id, title: id,
+                      start: now.addingTimeInterval(startOffset),
+                      end: now.addingTimeInterval(startOffset + duration),
+                      isAllDay: false, colorHex: "#4285f4", location: "", calendarKey: "local:1")
+    }
+
+    private static func snapshot(_ events: [SnapshotEvent]) -> CalendarrSnapshot {
+        CalendarrSnapshot(writtenAt: now,
+                          coverageStart: now.addingTimeInterval(-86400),
+                          coverageEnd: now.addingTimeInterval(14 * 86400),
+                          isLoggedIn: true, writerVersion: "1.0", events: events,
+                          theme: SnapshotTheme(today: "#1", text: "#2", background: "#3",
+                                               line: "#4", primary: "#5", accent: "#6"),
+                          language: "de")
+    }
+
+    @Test("The first entry is always now, so the complication renders immediately")
+    func startsAtNow() {
+        let dates = ComplicationTimeline.entryDates(from: Self.now, in: Self.snapshot([]))
+        #expect(dates.first == Self.now)
+    }
+
+    @Test("Entries are sorted and free of duplicates")
+    func sortedAndDeduplicated() {
+        let snap = Self.snapshot([Self.event("a", startOffset: 3600, duration: 1800),
+                                  Self.event("b", startOffset: 5400, duration: 1800)])
+        let dates = ComplicationTimeline.entryDates(from: Self.now, in: snap)
+
+        #expect(dates == dates.sorted())
+        #expect(Set(dates).count == dates.count)
+    }
+
+    @Test("Event starts and ends inside the horizon become entries")
+    func includesEventBoundaries() {
+        let snap = Self.snapshot([Self.event("a", startOffset: 3600, duration: 1800)])
+        let dates = ComplicationTimeline.entryDates(from: Self.now, in: snap)
+
+        #expect(dates.contains(Self.now.addingTimeInterval(3600)))
+        #expect(dates.contains(Self.now.addingTimeInterval(5400)))
+    }
+
+    @Test("Boundaries outside the horizon are ignored")
+    func ignoresBoundariesBeyondHorizon() {
+        let snap = Self.snapshot([Self.event("far", startOffset: 40 * 3600, duration: 1800)])
+        let dates = ComplicationTimeline.entryDates(from: Self.now, in: snap, horizon: 24 * 3600)
+
+        #expect(dates.contains(Self.now.addingTimeInterval(40 * 3600)) == false)
+        #expect(dates.allSatisfy { $0 <= Self.now.addingTimeInterval(24 * 3600) })
+    }
+
+    @Test("Boundaries already in the past are ignored")
+    func ignoresPastBoundaries() {
+        let snap = Self.snapshot([Self.event("past", startOffset: -7200, duration: 1800)])
+        let dates = ComplicationTimeline.entryDates(from: Self.now, in: snap)
+
+        #expect(dates.allSatisfy { $0 >= Self.now })
+    }
+
+    @Test("An empty calendar still gets hourly entries across the horizon")
+    func fillsHourlyWhenEmpty() {
+        let dates = ComplicationTimeline.entryDates(from: Self.now, in: Self.snapshot([]),
+                                                    horizon: 24 * 3600)
+        #expect(dates.count >= 24)
+    }
+
+    @Test("The entry count is capped, keeping the earliest entries")
+    func capsEntries() {
+        var events: [SnapshotEvent] = []
+        for i in 0..<300 {
+            events.append(Self.event("e\(i)", startOffset: Double(i) * 60, duration: 30))
+        }
+        let dates = ComplicationTimeline.entryDates(from: Self.now, in: Self.snapshot(events),
+                                                    maxEntries: 50)
+
+        #expect(dates.count == 50)
+        #expect(dates.first == Self.now)
+        #expect(dates == dates.sorted())
+    }
+}