class Date

def beginning_of_week(start_day = :monday)

have their time set to 0:00.
assumed to start on +start_day+, default is +:monday+. +DateTime+ objects
Returns a new +Date+/+DateTime+ representing the start of this week. Week is 
def beginning_of_week(start_day = :monday)
  days_to_start = days_to_week_start(start_day)
  result = self - days_to_start
  acts_like?(:time) ? result.midnight : result
end