if user write only in comment then write current datetime in field.

_inherit = "res.partner"

_columns = {

'comment_date' : fields.datetime("comment_date")

}

def write(self, cr, uid, ids,vals, context=None):

    d = datetime.now().date()

    currentdate=time.strftime('%Y-%m-%d')

    partner_obj = self.pool.get('res.partner')

    res = super(res_partner, self).write(cr, uid, ids,vals, context=context)

    if vals['comment']:

    p_id = partner_obj.browse(cr, uid,ids, context=context)

    partner_obj.write(cr, uid,p_id, {'comment_date': currentdate},context=context)

return res