dm.rb

require 'date'

def three_months_from(d)
    return d >> 3
end

test_dm.rb

require './dm'
require 'test/unit'

class TestThreeMonthsFromDate < Test::Unit::TestCase
    def _test_date(input_params, output_params)
        target = DateTime.new(*input_params)
        assert_equal(three_months_from(target), DateTime.new(*output_params))
    end

    def test20130331
        _test_date([2013, 3, 31], [2013, 6, 30])
    end

    def test20130401
        _test_date([2013, 4, 1], [2013, 7, 1])
    end

    def test20131129
        _test_date([2013, 11, 29], [2014, 2, 28])
    end

    def test20131130
        _test_date([2013, 11, 30], [2014, 2, 28])
    end

    def test20151129
        _test_date([2015, 11, 29], [2016, 2, 29])
    end

    def test20151130
        _test_date([2015, 11, 30], [2016, 2, 29])
    end
end

# Result:
#
# Loaded suite test_dm
# Started
# ......
# Finished in 0.002752 seconds.
#
# 6 tests, 6 assertions, 0 failures, 0 errors