class RubyXL::Workbook

def date_to_num(date)
  return nil if date.nil?
  if @date1904
    compare_date = DateTime.parse('December 31, 1903')
  else
    compare_date = DateTime.parse('December 31, 1899')
  end
  # add one day to compare date for erroneous 1900 leap year compatibility
  date.ajd + 1 - compare_date.ajd
end