The Ruby Way: Solutions and Techniques in Ruby Programming, Second Edition

7.23. Determining the Number of Days in a Month

At the time of this writing, there is no built-in function to do this. You can easily write a simple method for this:

require 'date'
def month_days(month,year=Date.today.year)
  mdays = [nil,31,28,31,30,31,30,31,31,30,31,30,31]
  mdays[2] = 29 if Date.leap?(year)
  mdays[month]
end

days = month_days(5)             # 31 (May)
days = month_days(2,2000)        # 29 (February 2000)
days = month_days(2,2100)        # 28 (February 2000)