Time stamp can not be obtained in the test instance

I use follow structure to obtain time stamp on the PX2, but captureGlobalTimeStamp is 0, and captureTimeStamp is also. How can I get the timestamp of frame when I get a frame of data?

typedef struct {

/*! Image capture time-stamp. /
NvMediaTime captureTimeStamp;
! Image capture global timestamp in microseconds. */
NvMediaGlobalTime captureGlobalTimeStamp;
} NvMediaImage;

Dear jingbin,

Could you please help to refer to below for this topic? Thanks.