diff --git a/PulseLoop/RingProtocol/ColmiDecoder.swift b/PulseLoop/RingProtocol/ColmiDecoder.swift
index 785a8e2..3dd257c 100644
--- a/PulseLoop/RingProtocol/ColmiDecoder.swift
+++ b/PulseLoop/RingProtocol/ColmiDecoder.swift
@@ -211,8 +211,13 @@ struct ColmiDecoder {
         comps.year = 2000 + hexLit(v[1])
         comps.month = hexLit(v[2])
         comps.day = hexLit(v[3])
-        comps.hour = min(23, max(0, Int(v[4]) / 4))   // nth quarter of day, clamped to a valid hour
-        comps.minute = 0
+        // v[4] is the nth quarter-hour of the day (0…95): hour = q/4, minute = (q%4)*15. Dropping the
+        // within-hour offset collapsed all four quarters of an hour onto HH:00, so — since the day
+        // total is the sum of buckets keyed by their start epoch — three of every four quarters
+        // overwrote each other and daily steps/distance were undercounted by up to ~75%.
+        let quarter = min(95, max(0, Int(v[4])))
+        comps.hour = quarter / 4
+        comps.minute = (quarter % 4) * 15
         comps.second = 0
         guard let ts = calendar.date(from: comps) else { return [] }
         // Reject implausible dates (misframed packet / bad BCD): must be within the last ~8 days.
diff --git a/PulseLoopTests/ColmiDecoderTests.swift b/PulseLoopTests/ColmiDecoderTests.swift
index 7818ea8..b78b577 100644
--- a/PulseLoopTests/ColmiDecoderTests.swift
+++ b/PulseLoopTests/ColmiDecoderTests.swift
@@ -53,6 +53,34 @@ final class ColmiDecoderTests: XCTestCase {
         XCTAssertEqual(percent, 84)
     }
 
+    // MARK: Activity history buckets
+
+    /// Two quarter-hour buckets in the same hour must decode to distinct timestamps (:30 vs :45).
+    /// Regression: the decoder used to hard-code minute 0, collapsing all four quarters of an hour
+    /// onto HH:00; since day totals sum buckets keyed by start epoch, three of four quarters
+    /// overwrote each other and steps/distance were undercounted by up to ~75%.
+    func testActivityHistoryPreservesQuarterHourOffset() {
+        // Reference clock so the decoded timestamp lands inside the [now-8d, now+1h] window.
+        let now = calendar.date(from: DateComponents(year: 2026, month: 7, day: 13, hour: 12))!
+        // BCD-literal date bytes for 2026-07-13, then the quarter-of-day index and steps=u16(v9,v10).
+        func frame(quarter: UInt8, steps: UInt8) -> Data {
+            ColmiPacket.frame([ColmiCommandID.syncActivity, 0x26, 0x07, 0x13, quarter,
+                               0, 0, 0, 0, steps, 0, 0, 0])
+        }
+        guard case let .activityBucket(ts6, s6, _) =
+                decoder.decodeHistory(frame(quarter: 6, steps: 100), day: now, calendar: calendar, now: now).first,
+              case let .activityBucket(ts7, s7, _) =
+                decoder.decodeHistory(frame(quarter: 7, steps: 200), day: now, calendar: calendar, now: now).first else {
+            return XCTFail("expected activityBucket events")
+        }
+        XCTAssertEqual(s6, 100)
+        XCTAssertEqual(s7, 200)
+        XCTAssertEqual(calendar.component(.hour, from: ts6), 1)
+        XCTAssertEqual(calendar.component(.minute, from: ts6), 30)  // quarter 6 → 01:30
+        XCTAssertEqual(calendar.component(.minute, from: ts7), 45)  // quarter 7 → 01:45
+        XCTAssertNotEqual(ts6, ts7, "same-hour quarter buckets must not share a timestamp/epoch")
+    }
+
     func testRealtimeHeartRate() {
         let frame = ColmiPacket.frame([ColmiCommandID.realtimeHeartRate, 72])
         let events = decoder.decodeNormal(frame)