require 'date'
# Class for 5-4-4, 4-5-4, and 4-4-5 calendars
class Calendar13
def initialize (type = 454, yr_end_mo = 1)
case type
when 544, 454, 445 then @type = type
else raise ArgumentError, "Bad calendar type #{type}"
end
if (1..12) === yr_end_mo then @yr_end_mo = yr_end_mo
else raise ArgumentError, "Bad year-end month #{yr_end_mo}"
end
end
# Return the ending date for a particular year
def end_of_year (year)
year += 1 unless @yr_end_mo == 12
year_end = Date.new year, @yr_end_mo, -1
wday = (year_end.wday + 1) % 7 # Saturday-origin day-of-week
# Advance or retreat to closest Saturday
if wday > 3 then year_end += 7 - wday
elsif wday > 0 then year_end -= wday
end
year_end
end
# Return starting date for a particular year
def start_of_year (year); end_of_year(year - 1) + 1; end
# Return starting date for a particular month
def start_of_month (year, month)
start = start_of_year(year) + ((month - 1) / 3).to_i * 91
case @type * 10 + (month - 1) % 3
when 4451, 4541 then start += 28
when 5441 then start += 35
when 4452 then start += 56
when 4542, 5442 then start += 63
end
start
end
# Return the ending date for a particular month
def end_of_month (year, month)
if month == 12 then end_of_year year
else start_of_month(year, month + 1) - 1
end
end
# Return the starting date for a particular quarter
def start_of_quarter (year, quarter)
start_of_month year, (quarter - 1) * 3 + 1
end
# Return the ending date for a particular quarter
def end_of_quarter (year, quarter)
if quarter == 4 then end_of_year year
else start_of_quarter(year, quarter + 1) - 1
end
end
# Return the number of weeks in a particular year
def weeks_in_year (year)
((start_of_year(year + 1) - start_of_year(year)) / 7).to_i
end
end
See also http://www.smythretail.com/general-retailing/how-to-set-up-a-4-5-4-calendar/.