Add local time characteristic (#4)

Co-authored-by: cybuzuma <cybuzuma@vnxs.de>
Co-committed-by: cybuzuma <cybuzuma@vnxs.de>

infinitime.go

@@ -33,6 +33,7 @@ const (
 	MotionValChar   = "00030002-78fc-48fe-8e23-433b3a1942d0"
 	FirmwareVerChar = "00002a26-0000-1000-8000-00805f9b34fb"
 	CurrentTimeChar = "00002a2b-0000-1000-8000-00805f9b34fb"
+	LocalTimeChar   = "00002a0f-0000-1000-8000-00805f9b34fb"
 	BatteryLvlChar  = "00002a19-0000-1000-8000-00805f9b34fb"
 	HeartRateChar   = "00002a37-0000-1000-8000-00805f9b34fb"
 	FSTransferChar  = "adaf0200-4669-6c65-5472-616e73666572"
@@ -47,6 +48,7 @@ var charNames = map[string]string{
 	MotionValChar:    "Motion Values",
 	FirmwareVerChar:  "Firmware Version",
 	CurrentTimeChar:  "Current Time",
+	LocalTimeChar:    "Local Time",
 	BatteryLvlChar:   "Battery Level",
 	HeartRateChar:    "Heart Rate",
 	FSTransferChar:   "Filesystem Transfer",
@@ -66,6 +68,7 @@ type Device struct {
 	motionValChar   *gatt.GattCharacteristic1
 	fwVersionChar   *gatt.GattCharacteristic1
 	currentTimeChar *gatt.GattCharacteristic1
+	localTimeChar   *gatt.GattCharacteristic1
 	battLevelChar   *gatt.GattCharacteristic1
 	heartRateChar   *gatt.GattCharacteristic1
 	fsVersionChar   *gatt.GattCharacteristic1
@@ -421,6 +424,8 @@ func (i *Device) resolveChars() error {
 			i.fwVersionChar = char
 		case CurrentTimeChar:
 			i.currentTimeChar = char
+		case LocalTimeChar:
+			i.localTimeChar = char
 		case BatteryLvlChar:
 			i.battLevelChar = char
 		case HeartRateChar:
@@ -708,7 +713,9 @@ func (i *Device) WatchMotion(ctx context.Context) (<-chan MotionValues, error) {
 	return out, nil
 }
 
-// SetTime sets the watch's time using the Current Time Service
+// SetTime sets the watch's
+// * time using the Current Time Service's current time characteristic
+// * timezone information using the CTS's local time characteristic
 func (i *Device) SetTime(t time.Time) error {
 	if err := i.checkStatus(i.currentTimeChar, CurrentTimeChar); err != nil {
 		return err
@@ -723,7 +730,38 @@ func (i *Device) SetTime(t time.Time) error {
 	binary.Write(buf, binary.LittleEndian, uint8(t.Weekday()))
 	binary.Write(buf, binary.LittleEndian, uint8((t.Nanosecond()/1000)/1e6*256))
 	binary.Write(buf, binary.LittleEndian, uint8(0b0001))
-	return i.currentTimeChar.WriteValue(buf.Bytes(), nil)
+	if err := i.currentTimeChar.WriteValue(buf.Bytes(), nil); err != nil {
+		return err
+	}
+
+	if err := i.checkStatus(i.localTimeChar, LocalTimeChar); err != nil {
+		// If the characteristic is unavailable,
+		// fail silently, as many people may be on
+		// older InfiniTime versions. A warning
+		// may be added later.
+		if _, ok := err.(ErrCharNotAvail); ok {
+			return nil
+		} else {
+			return err
+		}
+	}
+	_, offset := t.Zone()
+	dst := 0
+
+	// Local time expects two values: the timezone offset and the dst offset, both
+	// expressed in quarters of an hour.
+	// Timezone offset is to be constant over DST, with dst offset holding the offset != 0
+	// when DST is in effect.
+	// As there is no standard way in go to get the actual dst offset, we assume it to be 1h
+	// when DST is in effect
+	if t.IsDST() {
+		dst = 3600
+		offset -= 3600
+	}
+	bufTz := &bytes.Buffer{}
+	binary.Write(bufTz, binary.LittleEndian, uint8(offset / 3600 * 4))
+	binary.Write(bufTz, binary.LittleEndian, uint8(dst / 3600 * 4))
+	return i.localTimeChar.WriteValue(bufTz.Bytes(), nil)
 }
 
 // Notify sends a notification to InfiniTime via