'Return date and time (from to from) based on duration with timezone in string. Eg :
1) if user add duration for 2 hours, return : August-23-2013 at (04-30 To 06-30) (Europe/Brussels)
2) if event all day ,return : AllDay, July-31-2013'

@api.model
def _get_display_time(self, start, stop, zduration, zallday):

    timezone = self._context.get('tz')
    if (not timezone):
        timezone = (self.env.user.partner_id.tz or 'UTC')
    timezone = tools.ustr(timezone).encode('utf-8')
    (format_date, format_time) = self.with_context(tz=timezone)._get_date_formats()
    date = fields.Datetime.context_timestamp(self.with_context(tz=timezone), fields.Datetime.from_string(start))
    date_deadline = fields.Datetime.context_timestamp(self.with_context(tz=timezone), fields.Datetime.from_string(stop))
    date_str = date.strftime(format_date)
    time_str = date.strftime(format_time)
    if zallday:
        display_time = (_('AllDay    ,    %s') % date_str)
    elif (zduration < 24):
        duration = (date + timedelta(hours=zduration))
        display_time = (_('%s    at    (%s    To    %s)    (%s)') % (date_str, time_str, duration.strftime(format_time), timezone))
    else:
        display_time = (_('%s    at    %s    To\n    %s    at    %s    (%s)') % (date_str, time_str, date_deadline.strftime(format_date), date_deadline.strftime(format_time), timezone))
    return display_time
