module DateAndTime::Calculations

def days_to_week_start(start_day = Date.beginning_of_week)

+Date.beginning_of_week+ or +config.beginning_of_week+ when set.
Week is assumed to start on +start_day+, default is
Returns the number of days to the start of the week on the given day. 
def days_to_week_start(start_day = Date.beginning_of_week)
  start_day_number = DAYS_INTO_WEEK.fetch(start_day)
  (wday - start_day_number) % 7
end