TIME RANGES - Mailing list pgsql-sql

hi!
the only place I managed to find where time ranges are mentioned is
"pgbuilti"n man pages:
================
TIME RANGES
       Time ranges are specified as:

       [ 'abstime' 'abstime']
       where  abstime  is  a  time in the absolute time format.  Special
abstime
       values such as "current", "infinity" and "-infinity" can be used.
============================
So, the question is: how can one use them

Thanks

Aleksey