def is_top_down(lines):
date_lines = [<EOL>line for line in lines if hasattr(line, '<STR_LIT>') and line.is_date_line<EOL>]<EOL>if len(date_lines) < <NUM_LIT:2> or date_lines[<NUM_LIT:0>].date == date_lines[<NUM_LIT:1>].date:<EOL><INDENT>return None<EOL><DEDENT>else:<EOL><INDENT>return date_lines[<NUM_LIT:1>].date > date_lines[<NUM_LIT:0>].date<EOL><DEDENT>
Return `True` if dates in the given lines go in an ascending order, or `False` if they go in a descending order. If
no order can be determined, return `None`. The given `lines` must be a list of lines, ie.
:class:`~taxi.timesheet.lines.TextLine`, :class:`taxi.timesheet.lines.Entry` or
:class:`~taxi.timesheet.lines.DateLine`.