Here is an overview of the various timelines used in GStreamer.
The image below represents the different times in the pipeline when playing a 100ms sample and repeating the part between 50ms and 100ms.
You can see how the running-time of a buffer always increments monotonically along with the clock-time. Buffers are played when their running-time is equal to the clock-time - base-time. The stream-time represents the position in the stream and jumps backwards when repeating.