from datetime import datetime


def strptime(timestring):
    """
    Parse a datetime from an XPT format string.

    All text in an XPT document are ASCII-encoded.  This function
    expects a bytes string in the "ddMMMyy:hh:mm:ss" format.  For
    example, ``b'16FEB11:10:07:55'``.  Note that XPT supports only
    2-digit years, which are expected to be either 1900s or 2000s.
    """
    text = timestring.decode('ascii')
    return datetime.strptime(text, '%d%b%y:%H:%M:%S')