I needed a filter that would show deliver orders that were due today or late. Here’s how I accomplished that:

<filter name=“today” string=“For Today” 
        domain=“[(‘state’,'in’,('assigned’, 'partially_available’)),
                 ('min_date’, ’&lt;=’,datetime.datetime.now().strftime(’%Y-%m-%d 23:23:59’))]” 
        help=“Pickings to be done today”/>

I've written a post that goes into a bit more detail here: http://haplesstechnoweenie.com/post/143806280450/add-for-today-date-filter-to-odoo-search-view