Easy way to determine leap year in ruby?

Here is my answer for the exercism.io problem which asks the same question. You are explicitly told to ignore any standard library functions that may implement it as part of the exercise.

class Year
  attr_reader :year

  def initialize(year)
    @year = year
  end

  def leap?
    if @year.modulo(4).zero?
      return true unless @year.modulo(100).zero? and not @year.modulo(400).zero?
    end

    false
  end
end

Any year that is evenly divisible by 4 is a leap year. However, there is still a small error that must be accounted for. To eliminate this error, the Gregorian calendar stipulates that a year that is evenly divisible by 100 (for example, 1900) is a leap year only if it is also evenly divisible by 400.

class Date

  def self.leap?(year)
    year % 4 == 0 && year % 100 != 0 || year % 400 == 0
  end

end

Using the least amount possible of comparisons, you could do this:

• First/Longer version

def leap_year?(year)
  # check first if year is divisible by 400
  return true if year % 400 == 0
  year % 4 == 0 && year % 100 != 0
end

• Shorter version

We can do the same check using short circuit OR (||):

def leap_year?(year)
  year % 400 == 0 || (year % 4 == 0 && year % 100 != 0)
end

Already a lot of answers, though here is a shorter version:

y%100 > 0 ? y%4 < 1 : y%400 < 1

If the year is not a century, then checks if it is a multiple of 4, otherwise checks if it is multiple of 400.

Here's my method:

def leap_year?(year)
  Time.new(year, 12, 31).yday == 366
end

leap_year?(1999)
#=>  false

leap_year?(2000)
#=>  true

leap_year?(2100)
#=>  false

leap_year?(2104)
#=>  true