Hi,
buf_pts is generated in gstreamer framework. If you use nvarguscamerasrc plugin, you can get the timestamp while frames are being captured in kernel. The information is in
typedef struct AuxiliaryData {
gint64 frame_num;
gint64 timestamp;
} AuxData;
We have nvarguscamerasrc open source. You can look at the source code for more information. This may be more accurate and can be applied in your usecase.