weeknumber.rb

# Trivial example: 2017-01-01 is part of the last week of 2016 because ISO week starts with the Monday.
irb(main):004:0> fd2017 = Date.new(2017,1,1)
=> #<Date: 2017-01-01 ((2457755j,0s,0n),+0s,2299161j)>
irb(main):005:0> fd2017.cweek
=> 52
irb(main):006:0> fd2017.cwday
=> 7 # Sunday
irb(main):007:0> fd2017.cwyear
=> 2016

# What if the years with Tuesday?
irb(main):008:0> fd2019 = Date.new(2019,1,1)
=> #<Date: 2019-01-01 ((2458485j,0s,0n),+0s,2299161j)>
irb(main):009:0> fd2019.cwyear
=> 2019 # It's already in 2019!!
irb(main):010:0> fd2019.cwday
=> 2 # Tuesday

# Then how about the last day of 2018?
irb(main):011:0> ld2018 = Date.new(2018,12,31)
==> #<Date: 2018-12-31 ((2458484j,0s,0n),+0s,2299161j)>
irb(main):012:0> ld2018.cwday
=> 1 # Monday
irb(main):013:0> ld2018.cwyear
=> 2019 # Already in 2019!!
irb(main):014:0> ld2018.cweek
=> 1