datestring and datestring_gmt convert
    timestamps or datetimes
    from internal to external date-time representation.  The internal
    representation is an 8 byte binary string
    of the special type TIMESTAMP_OBJ, documented elsewhere
    and the external representation is a human-readable ASCII string of
    up to 30 characters. The external format is:
    YYYY-MM-DD hh:mm:ss uuuuuu
    where uuuuuu represents
    microseconds.