CC1352P: zstack_afIncomingMsgInd_t timestamp units

What are the time units on the timestamp that comes along with a zstackmsg_CmdIDs_AF_INCOMING_MSG_IND. I'd like to use this to determine the elapsed time between updates.

    int_least8_t rssi;
    /** Receipt timestamp from MAC */
    uint32_t timestamp;  
    /** Network header frame sequence number */
    uint8_t nwkSeqNum;
    /** MAC header source short address */
    uint16_t macSrcAddr;
    /** Transaction sequence number */
    uint8_t transSeqNum;
    /** Message's network radius, 0 mean internal */
    uint8_t radius;
    /** Length of the message "payload" */
    uint8_t n_payload;
    /** Pointer to the message "payload" */
    uint8_t *pPayload;
} zstack_afIncomingMsgInd_t;

Thanks,

- Bill