diff --git a/Sources/ICalGuyKit/Services/DateGrouper.swift b/Sources/ICalGuyKit/Services/DateGrouper.swift
index c32d09e..ffcd9bc 100644
--- a/Sources/ICalGuyKit/Services/DateGrouper.swift
+++ b/Sources/ICalGuyKit/Services/DateGrouper.swift
@@ -20,24 +20,12 @@ public struct DateGrouper: Sendable {
 
     for event in events {
       let eventStart = calendar.startOfDay(for: event.startDate)
-      let eventEnd = calendar.startOfDay(for: event.endDate)
 
-      let effectiveEnd: Date
-      if event.isAllDay {
-        // All-day events: EventKit sets endDate to midnight of the *next* day.
-        // Subtract 1 day so a 1-day event stays in 1 bucket, a 3-day event spans 3 days.
-        effectiveEnd = calendar.date(byAdding: .day, value: -1, to: eventEnd) ?? eventEnd
-      } else {
-        // Timed events ending exactly at midnight: don't spill into the next day
-        let endComponents = calendar.dateComponents([.hour, .minute, .second], from: event.endDate)
-        if endComponents.hour == 0 && endComponents.minute == 0 && endComponents.second == 0
-          && eventEnd > eventStart
-        {
-          effectiveEnd = calendar.date(byAdding: .day, value: -1, to: eventEnd) ?? eventEnd
-        } else {
-          effectiveEnd = eventEnd
-        }
-      }
+      // Bucket by the last instant the event occupies. EventKit ends all-day events at
+      // 23:59:59 of the last day (not midnight of the next), and timed events ending exactly
+      // at midnight shouldn't spill into the next day. Zero-duration events stay on their start day.
+      let lastInstant = max(event.startDate, event.endDate.addingTimeInterval(-1))
+      let effectiveEnd = calendar.startOfDay(for: lastInstant)
 
       var day = eventStart
       // Clip to query range boundaries
diff --git a/Tests/ICalGuyKitTests/DateGrouperTests.swift b/Tests/ICalGuyKitTests/DateGrouperTests.swift
index cf24057..c0705ab 100644
--- a/Tests/ICalGuyKitTests/DateGrouperTests.swift
+++ b/Tests/ICalGuyKitTests/DateGrouperTests.swift
@@ -170,6 +170,51 @@ final class DateGrouperTests: XCTestCase {
     XCTAssertEqual(groups[0].events.count, 1)
   }
 
+  // Real EventKit ends all-day events at 23:59:59 of the last day, not midnight of the next.
+
+  func testSingleDayAllDayEventWithEventKitEndDate() {
+    let events = [
+      makeEvent(
+        id: "1", title: "Holiday",
+        startDate: date(2024, 3, 15), endDate: date(2024, 3, 16).addingTimeInterval(-1),
+        isAllDay: true)
+    ]
+
+    let groups = grouper.groupByDate(events, from: date(2024, 3, 1), to: date(2024, 3, 31))
+
+    XCTAssertEqual(groups.count, 1)
+    XCTAssertEqual(groups[0].date, "2024-03-15")
+    XCTAssertEqual(groups[0].events.count, 1)
+  }
+
+  func testMultiDayAllDayEventWithEventKitEndDateIncludesLastDay() {
+    // Mar 15-17 inclusive
+    let events = [
+      makeEvent(
+        id: "1", title: "Conference",
+        startDate: date(2024, 3, 15), endDate: date(2024, 3, 18).addingTimeInterval(-1),
+        isAllDay: true)
+    ]
+
+    let groups = grouper.groupByDate(events)
+
+    XCTAssertEqual(groups.map(\.date), ["2024-03-15", "2024-03-16", "2024-03-17"])
+  }
+
+  func testAllDayEventWithEventKitEndDateClippedToRange() {
+    // Mar 14-17 inclusive, queried Mar 15-16
+    let events = [
+      makeEvent(
+        id: "1", title: "Conference",
+        startDate: date(2024, 3, 14), endDate: date(2024, 3, 18).addingTimeInterval(-1),
+        isAllDay: true)
+    ]
+
+    let groups = grouper.groupByDate(events, from: date(2024, 3, 15), to: date(2024, 3, 16))
+
+    XCTAssertEqual(groups.map(\.date), ["2024-03-15", "2024-03-16"])
+  }
+
   func testTimedOvernightEventSpansTwoDays() {
     // Timed event from 10pm Mar 15 to 2am Mar 16
     let events = [