CC2530数据包timestamp是用什么单位记录时间的？

cc2530 Z-stack中收到的数据包格式为afIncomingMSGPacket_t ，我想知道其中uint32 timestamp; /* receipt timestamp from MAC */  所记录的mac层时间具体在哪里产生并在什么情况下记录的？并且我查看了所记录的时间，其单位不是1ms 也不是1μs，那么到底是什么时间单位记录的。。
另外这个时间能否作为节点的本地时间？又该如何调整更改呢？因为我要做节点时间同步的问题。数据结构如下
typedef struct
{
  osal_event_hdr_t hdr; /* OSAL Message header */
  uint16 groupId; /* Message's group ID - 0 if not set */
  uint16 clusterId; /* Message's cluster ID */
  afAddrType_t srcAddr; /* Source Address, if endpoint is STUBAPS_INTER_PAN_EP,
  it's an InterPAN message */
  uint16 macDestAddr; /* MAC header destination short address */
  uint8 endPoint; /* destination endpoint */
  uint8 wasBroadcast; /* TRUE if network destination was a broadcast address */
  uint8 LinkQuality; /* The link quality of the received data frame */
  uint8 correlation; /* The raw correlation value of the received data frame */
  int8 rssi; /* The received RF power in units dBm */
  uint8 SecurityUse; /* deprecated */
  uint32 timestamp; /* receipt timestamp from MAC */
  uint8 nwkSeqNum; /* network header frame sequence number */
  afMSGCommandFormat_t cmd; /* Application Data */
} afIncomingMSGPacket_t;