Determining the Number of Days In a Month

def days_in_month(year, month)
  Date.new(year, 12, 31).prev_month(12 - month).day
end

You’re basically grabbing the last day of the year, and then getting the day for n months previous (12 - month).

Update: For those who ask, “Yeah, but how do you find the first day of the month?” It’s “1”.