diff --git a/lib/openstrap_protocol.dart b/lib/openstrap_protocol.dart
index ee87030..403066e 100644
--- a/lib/openstrap_protocol.dart
+++ b/lib/openstrap_protocol.dart
@@ -18,6 +18,7 @@ export 'src/band.dart' show DeviceType, GattProfile, BandProfile;
 // sharing one barrel must not share a bare verb.
 export 'src/oura.dart';
 export 'src/hrs.dart';
+export 'src/dafit.dart';
 export 'src/polar_pmd.dart';
 export 'src/lefun.dart';
 
diff --git a/lib/src/dafit.dart b/lib/src/dafit.dart
new file mode 100644
index 0000000..193b199
--- /dev/null
+++ b/lib/src/dafit.dart
@@ -0,0 +1,192 @@
+// DaFit / MOYOUNG-V2 wire format — the framing spoken by a large cluster of
+// unbranded Chinese OEM watches, paired through the "DaFit" or "MOYOUNG"
+// companion app and sold under dozens of storefront names. Bytes only: no
+// BLE, no Flutter, no database.
+//
+// NOTHING HERE HAS MET HARDWARE. Ships EXPERIMENTAL (ASSUMPTIONS R6): this
+// file decodes NO physiological signal from this family's frames, only the
+// framing needed to hold a session open and bank what the band sends. A
+// data-bearing reply that goes unacknowledged is documented to leave the
+// band stalled and draining its battery fast, so the minimal ack this file
+// builds is protocol plumbing, not a derived metric — nothing here turns
+// bytes into a heart rate, a step count or a sleep stage.
+//
+// TRANSPORT is an otherwise-generic Nordic UART Service link (one write
+// characteristic, one notify characteristic, no CRC, no encryption).
+//
+// DATA FRAME (header 0xCD), app<->band, big-endian length fields:
+//   [0]     header (0xCD)
+//   [1..2]  outer length: byte count from [3] to the end of payload,
+//           i.e. 5 + payload.length
+//   [3]     command group
+//   [4]     protocol version/delimiter — always 0x01
+//   [5]     command
+//   [6..7]  payload length
+//   [8..]   payload
+//
+// ACK FRAME (header 0xDC), a fixed 8 bytes, sent to keep a data-bearing
+// exchange moving:
+//   [0] 0xDC  [1..2] 0x00 0x05 (fixed)  [3] command group  [4] 0x01
+//   [5..6] (outer length of the frame being acked) + 3
+//   [7] 0x01
+// This shape does not nest inside the data-frame layout above — it is
+// parsed and built separately, never through [parseDafitFrame].
+
+import 'dart:typed_data';
+
+const int kDafitDataHeader = 0xCD;
+const int kDafitAckHeader = 0xDC;
+
+const int kDafitGroupGeneral = 0x12;
+const int kDafitGroupRequestData = 0x1a;
+const int kDafitGroupBandInfo = 0x20;
+
+const int kDafitCmdSetDateTime = 0x01;
+const int kDafitCmdSetLanguage = 0x15;
+const int kDafitCmdInit1 = 0x0a;
+const int kDafitCmdInit2 = 0x0c;
+const int kDafitCmdInit3 = 0xff;
+
+/// group [kDafitGroupRequestData].
+const int kDafitCmdGetHwInfo = 0x10;
+
+/// group [kDafitGroupBandInfo].
+const int kDafitCmdGetBandInfo = 0x02;
+
+const int kDafitValueOn = 0x01;
+const int kDafitLangEnglish = 0x01;
+
+/// One parsed data frame ([kDafitDataHeader]). Null for anything else,
+/// including this family's own ACK frames — see [parseDafitFrame].
+class DafitFrame {
+  final int group;
+  final int command;
+  final Uint8List payload;
+
+  /// The frame's own declared outer length (bytes [1..2]) — kept verbatim,
+  /// not recomputed from [payload], so [buildDafitAck] echoes exactly what
+  /// the band itself declared rather than this file's own arithmetic.
+  final int outerLen;
+
+  const DafitFrame(this.group, this.command, this.payload, this.outerLen);
+}
+
+/// Parse one notification as a data frame. Null when it cannot be one —
+/// too short, the wrong header (an ACK frame reads `0xDC` here and is
+/// correctly rejected), a payload length longer than the bytes actually
+/// delivered, or an outer length that disagrees with the payload length —
+/// the two are supposed to describe the same frame (`outerLen == 5 +
+/// payloadLen`), and a frame where they disagree is malformed, not merely
+/// unfamiliar. Accepting it anyway would hand [buildDafitAck] a length to
+/// echo back that this frame never actually had.
+DafitFrame? parseDafitFrame(List<int> value) {
+  if (value.length < 8 || value[0] != kDafitDataHeader) return null;
+  final outerLen = (value[1] << 8) | value[2];
+  final payloadLen = (value[6] << 8) | value[7];
+  if (value.length - 8 < payloadLen || outerLen != 5 + payloadLen) {
+    return null;
+  }
+  return DafitFrame(
+    value[3],
+    value[5],
+    Uint8List.fromList(value.sublist(8, 8 + payloadLen)),
+    outerLen,
+  );
+}
+
+/// Build one outbound data frame.
+Uint8List buildDafitFrame(
+  int group,
+  int command, [
+  List<int> payload = const <int>[],
+]) {
+  final out = Uint8List(8 + payload.length);
+  final outerLen = 5 + payload.length;
+  out[0] = kDafitDataHeader;
+  out[1] = (outerLen >> 8) & 0xff;
+  out[2] = outerLen & 0xff;
+  out[3] = group & 0xff;
+  out[4] = 0x01;
+  out[5] = command & 0xff;
+  out[6] = (payload.length >> 8) & 0xff;
+  out[7] = payload.length & 0xff;
+  out.setRange(8, 8 + payload.length, payload);
+  return out;
+}
+
+/// Build the fixed 8-byte ACK for a received data frame, so a data-bearing
+/// exchange keeps moving instead of stalling. Takes the frame being
+/// acknowledged directly — its [DafitFrame.outerLen] is the only input, so
+/// there is no separate length argument to drift out of sync with it.
+Uint8List buildDafitAck(DafitFrame acked) {
+  final ackSize = acked.outerLen + 3;
+  return Uint8List.fromList(<int>[
+    kDafitAckHeader, 0x00, 0x05, //
+    acked.group, 0x01,
+    (ackSize >> 8) & 0xff, ackSize & 0xff,
+    0x01,
+  ]);
+}
+
+/// True for the fixed 8-byte ACK shape ([kDafitAckHeader]) — the direction
+/// this family also uses band-to-app, interspersed with data frames during
+/// the same exchange. Not decoded further: nothing downstream needs more
+/// than "this notification was an ack, not data".
+///
+/// Checks every fixed byte the shape actually has ([1], [2], [4] and [7] are
+/// all constant — see [buildDafitAck]), not just the header: an 8-byte
+/// notification that happens to start with 0xDC but disagrees with the rest
+/// of the shape is not an ack, it is something this file does not recognise.
+bool isDafitAckFrame(List<int> value) =>
+    value.length == 8 &&
+    value[0] == kDafitAckHeader &&
+    value[1] == 0x00 &&
+    value[2] == 0x05 &&
+    value[4] == 0x01 &&
+    value[7] == 0x01;
+
+/// Pack a local date/time into this family's bit-packed SET_DATE_TIME field:
+/// seconds, then (year-2000), month, day, hour, minute each shifted into
+/// their own bit range of one big-endian u32.
+///
+/// `year - 2000` has to fit the field's own six bits (0-63, i.e. 2000-2063).
+/// Outside that range the later shift would silently drop the high bits and
+/// write a DIFFERENT year to the band's clock — a wrong date it would then
+/// act on with total confidence — so this throws instead.
+int packDafitDateTime(DateTime t) {
+  final yearOffset = t.year - 2000;
+  if (yearOffset < 0 || yearOffset > 63) {
+    throw ArgumentError.value(t.year, 'year',
+        'this family\'s clock field only represents 2000-2063');
+  }
+  return t.second |
+      (yearOffset << 26) |
+      (t.month << 22) |
+      (t.day << 17) |
+      (t.hour << 12) |
+      (t.minute << 6);
+}
+
+/// Handshake frames, in the order this family expects them. Sending them
+/// unconditionally is documented behaviour, not a guess: skipping this
+/// sequence is reported to leave later fetches refused and the band's
+/// battery draining fast, and each step is a control write, not a request
+/// for any physiological data. A caller pauses between writes; timing is a
+/// session concern and stays out of this file.
+List<Uint8List> dafitInitSequence(DateTime now) => <Uint8List>[
+      buildDafitFrame(kDafitGroupGeneral, kDafitCmdInit1, const [0x02]),
+      buildDafitFrame(
+        kDafitGroupGeneral,
+        kDafitCmdSetDateTime,
+        (ByteData(4)..setUint32(0, packDafitDateTime(now) & 0xffffffff))
+            .buffer
+            .asUint8List(),
+      ),
+      buildDafitFrame(kDafitGroupRequestData, kDafitCmdInit1),
+      buildDafitFrame(kDafitGroupRequestData, kDafitCmdInit2),
+      buildDafitFrame(
+          kDafitGroupGeneral, kDafitCmdSetLanguage, const [kDafitLangEnglish]),
+      buildDafitFrame(kDafitGroupGeneral, kDafitCmdInit3, const [kDafitValueOn]),
+      buildDafitFrame(kDafitGroupRequestData, kDafitCmdGetHwInfo),
+      buildDafitFrame(kDafitGroupBandInfo, kDafitCmdGetBandInfo),
+    ];
diff --git a/test/dafit_test.dart b/test/dafit_test.dart
new file mode 100644
index 0000000..ada4fef
--- /dev/null
+++ b/test/dafit_test.dart
@@ -0,0 +1,111 @@
+import 'package:openstrap_protocol/openstrap_protocol.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group('DaFit / MOYOUNG-V2 framing', () {
+    test('parses a real captured SET_DATE_TIME data frame', () {
+      // 0xCD 0x00 0x09 0x12 0x01 0x01 0x00 0x04 0xA5 0x83 0x73 0xDB
+      final f = parseDafitFrame(
+          [0xcd, 0x00, 0x09, 0x12, 0x01, 0x01, 0x00, 0x04, 0xa5, 0x83, 0x73, 0xdb]);
+      expect(f, isNotNull);
+      expect(f!.group, 0x12);
+      expect(f.command, 0x01);
+      expect(f.outerLen, 0x09);
+      expect(f.payload, [0xa5, 0x83, 0x73, 0xdb]);
+    });
+
+    test('rejects an ACK-headed frame as a data frame', () {
+      // dc 00 05 1a 01 00 0c 01
+      expect(parseDafitFrame([0xdc, 0x00, 0x05, 0x1a, 0x01, 0x00, 0x0c, 0x01]),
+          isNull);
+    });
+
+    test('rejects a truncated payload', () {
+      expect(parseDafitFrame([0xcd, 0x00, 0x09, 0x12, 0x01, 0x01, 0x00, 0x04, 0xa5]),
+          isNull);
+    });
+
+    test('rejects an outer length that disagrees with the payload length',
+        () {
+      // Same bytes as the real captured frame above, but the outer length
+      // (byte 2) claims 8 instead of the correct 9 — a malformed frame, not
+      // a shorter one, since the payload bytes and their own length field
+      // still say 4.
+      expect(
+          parseDafitFrame(
+              [0xcd, 0x00, 0x08, 0x12, 0x01, 0x01, 0x00, 0x04, 0xa5, 0x83, 0x73, 0xdb]),
+          isNull);
+    });
+
+    test('isDafitAckFrame identifies only the fixed 8-byte ack shape', () {
+      expect(isDafitAckFrame([0xdc, 0x00, 0x05, 0x1a, 0x01, 0x00, 0x0c, 0x01]),
+          isTrue);
+      expect(
+          isDafitAckFrame(
+              [0xcd, 0x00, 0x09, 0x12, 0x01, 0x01, 0x00, 0x04, 0xa5, 0x83, 0x73, 0xdb]),
+          isFalse);
+    });
+
+    test('isDafitAckFrame rejects an 8-byte 0xDC packet with the wrong fixed '
+        'bytes', () {
+      // Right header and length, but the trailing fixed byte is not the 0x01
+      // this shape always carries.
+      expect(isDafitAckFrame([0xdc, 0x00, 0x05, 0x1a, 0x01, 0x00, 0x0c, 0x00]),
+          isFalse);
+    });
+
+    test('buildDafitFrame round-trips through parseDafitFrame', () {
+      final built = buildDafitFrame(0x12, 0x0a, const [0x02]);
+      expect(built, [0xcd, 0x00, 0x06, 0x12, 0x01, 0x0a, 0x00, 0x01, 0x02]);
+      final parsed = parseDafitFrame(built);
+      expect(parsed!.group, 0x12);
+      expect(parsed.command, 0x0a);
+      expect(parsed.payload, [0x02]);
+    });
+
+    test('buildDafitFrame with no payload', () {
+      final built = buildDafitFrame(0x1a, 0x0a);
+      expect(built, [0xcd, 0x00, 0x05, 0x1a, 0x01, 0x0a, 0x00, 0x00]);
+    });
+
+    test('buildDafitAck echoes the acked frame\'s own outer length', () {
+      final acked = parseDafitFrame([0xcd, 0x00, 0x05, 0x1a, 0x01, 0x0a, 0x00, 0x00])!;
+      final ack = buildDafitAck(acked);
+      expect(ack, [0xdc, 0x00, 0x05, 0x1a, 0x01, 0x00, 0x08, 0x01]);
+    });
+
+    test('packDafitDateTime packs fields into the documented bit ranges', () {
+      final t = DateTime(2024, 3, 5, 14, 22, 37);
+      final v = packDafitDateTime(t);
+      expect(v & 0x3f, 37); // seconds, bits 0-5
+      expect((v >> 6) & 0x3f, 22); // minute
+      expect((v >> 12) & 0x1f, 14); // hour
+      expect((v >> 17) & 0x1f, 5); // day
+      expect((v >> 22) & 0xf, 3); // month
+      expect((v >> 26) & 0x3f, 24); // year - 2000
+    });
+
+    test('packDafitDateTime refuses a year outside 2000-2063', () {
+      expect(() => packDafitDateTime(DateTime(1999, 1, 1)),
+          throwsArgumentError);
+      expect(() => packDafitDateTime(DateTime(2064, 1, 1)),
+          throwsArgumentError);
+      // The boundary itself is fine.
+      expect(() => packDafitDateTime(DateTime(2063, 1, 1)), returnsNormally);
+    });
+
+    test('dafitInitSequence is eight well-formed, parseable frames', () {
+      final seq = dafitInitSequence(DateTime(2024, 3, 5, 14, 22, 37));
+      expect(seq, hasLength(8));
+      for (final frame in seq) {
+        expect(frame[0], kDafitDataHeader);
+        expect(parseDafitFrame(frame), isNotNull);
+      }
+      // The clock write really carries the packed value from above.
+      final clockFrame = parseDafitFrame(seq[1])!;
+      expect(clockFrame.group, kDafitGroupGeneral);
+      expect(clockFrame.command, kDafitCmdSetDateTime);
+      expect(clockFrame.payload.length, 4);
+    });
+  });
+}