def beginning_of_week(start_day = Date.beginning_of_week)
      result = days_ago(days_to_week_start(start_day))
      acts_like?(:time) ? result.midnight : result
    end
def beginning_of_week(start_day = Date.beginning_of_week)
      result = days_ago(days_to_week_start(start_day))
      acts_like?(:time) ? result.midnight : result
    end
Returns a new date/time representing the start of this week on the given day.
 Week is assumed to start on +start_day+, default is
 +Date.beginning_of_week+ or +config.beginning_of_week+ when set.
 +DateTime+ objects have their time set to 0:00.