diff --git a/PulseLoop/RingProtocol/ColmiEncoder.swift b/PulseLoop/RingProtocol/ColmiEncoder.swift
index 4bbec61..faf1969 100644
--- a/PulseLoop/RingProtocol/ColmiEncoder.swift
+++ b/PulseLoop/RingProtocol/ColmiEncoder.swift
@@ -96,9 +96,14 @@ struct ColmiEncoder {
         [ColmiCommandID.syncActivity, UInt8(clamping: daysAgo), 0x0f, 0x00, 0x5f, 0x01]
     }
 
-    /// HR history from a unix timestamp (seconds), little-endian in bytes 1-4.
-    func syncHeartRate(fromUnix seconds: Int) -> [UInt8] {
-        let ts = UInt32(truncatingIfNeeded: seconds)
+    /// HR history request (0x15), timestamp little-endian in bytes 1-4. The ring's RTC runs on local
+    /// wall time (see `setDateTime`), so its firmware keys history days by "local-time-as-UTC"
+    /// epochs. Send local midnight *plus* the UTC offset — matching GadgetBridge
+    /// (`getTimeInMillis() + ZONE_OFFSET + DST_OFFSET`) and colmi_r02_client (midnight with
+    /// tzinfo=UTC). A true UTC epoch misses the day slot and the ring replies "empty" for every day.
+    func syncHeartRate(dayStart: Date, timeZone: TimeZone = .current) -> [UInt8] {
+        let offset = timeZone.secondsFromGMT(for: dayStart)
+        let ts = UInt32(truncatingIfNeeded: Int(dayStart.timeIntervalSince1970) + offset)
         return [
             ColmiCommandID.syncHeartRate,
             UInt8(ts & 0xff),
diff --git a/PulseLoop/RingProtocol/ColmiSyncEngine.swift b/PulseLoop/RingProtocol/ColmiSyncEngine.swift
index 1d44ae4..d6badc9 100644
--- a/PulseLoop/RingProtocol/ColmiSyncEngine.swift
+++ b/PulseLoop/RingProtocol/ColmiSyncEngine.swift
@@ -224,8 +224,7 @@ final class ColmiSyncEngine: RingSyncEngine {
 
     private func requestHeartRate() {
         syncDay = dayStart(daysAgo)
-        let unix = Int(syncDay.timeIntervalSince1970)
-        writer?.enqueue(Data(encoder.syncHeartRate(fromUnix: unix)))
+        writer?.enqueue(Data(encoder.syncHeartRate(dayStart: syncDay)))
     }
 
     private func requestStress() {
diff --git a/PulseLoopTests/ColmiEncoderTests.swift b/PulseLoopTests/ColmiEncoderTests.swift
new file mode 100644
index 0000000..971efff
--- /dev/null
+++ b/PulseLoopTests/ColmiEncoderTests.swift
@@ -0,0 +1,45 @@
+import XCTest
+@testable import PulseLoop
+
+final class ColmiEncoderTests: XCTestCase {
+
+    private let encoder = ColmiEncoder()
+
+    /// Little-endian u32 from the request's timestamp bytes 1-4.
+    private func timestamp(_ bytes: [UInt8]) -> UInt32 {
+        UInt32(bytes[1]) | UInt32(bytes[2]) << 8 | UInt32(bytes[3]) << 16 | UInt32(bytes[4]) << 24
+    }
+
+    private func midnight(in zone: TimeZone, year: Int, month: Int, day: Int) -> Date {
+        var calendar = Calendar(identifier: .gregorian)
+        calendar.timeZone = zone
+        return calendar.date(from: DateComponents(year: year, month: month, day: day))!
+    }
+
+    // The ring's RTC runs on local wall time, so the 0x15 HR-history request must carry local
+    // midnight *as if it were UTC* (epoch + UTC offset), matching GadgetBridge / colmi_r02_client.
+    // A true UTC epoch makes the ring reply "empty" for every day (the R03 field bug).
+
+    func testSyncHeartRatePositiveOffset() {
+        let zone = TimeZone(identifier: "Asia/Singapore")!   // UTC+8, no DST
+        let day = midnight(in: zone, year: 2026, month: 7, day: 24)
+        let bytes = encoder.syncHeartRate(dayStart: day, timeZone: zone)
+        XCTAssertEqual(bytes[0], ColmiCommandID.syncHeartRate)
+        XCTAssertEqual(timestamp(bytes), UInt32(day.timeIntervalSince1970) + 8 * 3600)
+    }
+
+    func testSyncHeartRateNegativeOffsetDSTAware() {
+        let zone = TimeZone(identifier: "America/Los_Angeles")!   // UTC-7 in July (PDT)
+        let day = midnight(in: zone, year: 2026, month: 7, day: 24)
+        let bytes = encoder.syncHeartRate(dayStart: day, timeZone: zone)
+        XCTAssertEqual(zone.secondsFromGMT(for: day), -7 * 3600)
+        XCTAssertEqual(timestamp(bytes), UInt32(Int(day.timeIntervalSince1970) - 7 * 3600))
+    }
+
+    func testSyncHeartRateGMTIsIdentity() {
+        let zone = TimeZone(identifier: "GMT")!
+        let day = midnight(in: zone, year: 2026, month: 7, day: 24)
+        let bytes = encoder.syncHeartRate(dayStart: day, timeZone: zone)
+        XCTAssertEqual(timestamp(bytes), UInt32(day.timeIntervalSince1970))
+    }
+}