GstReferenceTimestampMeta can be used to attach alternative timestamps and possibly durations to a gstreamer.Buffer These are generally not according to the pipeline clock and could be e.g. the NTP timestamp when the media was captured.
The reference is stored as a gstreamer.Caps in reference. Examples of valid references would be "timestamp/x-drivername-stream" for timestamps that are locally generated by some driver named "drivername" when generating the stream, e.g. based on a frame counter, or "timestamp/x-ntp, host=pool.ntp.org, port=123" for timestamps based on a specific NTP server.