days_in_month.rb

# Need to find out how many days are in the current month?
#   leverage the power of Date#<<(n), which "rewinds" the date by n months.
#   it helpfully sets the day correctly if you rewind from the 31st, so rewind from 12/31.

days_in_current_month = ( Date.new(Time.now.year, 12, 31).to_date << ( 12 - Time.now.month ) ).day

# You could also make this a method call

def days_in_month(month_number)
  ( Date.new(Time.now.year, 12, 31).to_date << ( 12 - month_number ) ).day
end