date_conv.rb

def date_to_a(t)
  [
    "#{t.year}年",
    "#{t.month}月",
    "#{t.day}日",
    " #{t.hour}時#{t.min}分",
  ]
end

def date_conv(t1, t2)
  case t1
  when nil
    date_to_a(t2)
  when t2
    ["#{t2.hour}時#{t2.min}分"]
  else
    rest = date_to_a(t1).zip(date_to_a(t2)).drop_while do |s1, s2|
      s1 == s2
    end
    rest.map { |s1, s2| s2 }
  end
end

list = [Time.new(2000, 1, 1), Time.new(2000, 1, 1)]
results = list.zip([nil, *list]).map do |t2, t1|
  date_conv(t1, t2)
end

p results.map(&:join).map(&:strip)