a.rb

class SystemProcess
  def initialize(pid)
    @pid = pid
  end

  def started_at
    self.class.time_of_boot + started_seconds_after_boot
  end

  class << self
    def uptime
      `cat /proc/uptime`.split(" ").first.to_f
    end

    def time_of_boot
      Time.now - uptime
    end
    
  end

  private

  def started_seconds_after_boot
    `cat /proc/#{@pid}/stat`.split(" ")[21].to_i / 100.0
  end
end

puts Time.now
sleep 10
puts SystemProcess.new(Process.pid).started_at