Mime type for time-stamped text in gstreamer subtitleoverlay element

According to the documentation, the subtitleoverlay element supports timestamped text. Is there a standard for times tamped text supported by this element? If yes, what is the mime type and the expected format?

I was able to render raw text read from a file on top of the video using the following pipeline:

gst-launch-0.10 videotestsrc ! \
    subtitleoverlay name=ovl ! \
    autovideosink \
    filesrc location=test.txt ! text/plain !  ovl.

But this just renders static text, not dynamic text based on the time stamps.