Complex sentence: Recalling our two observers from different ages: the time in their experiments is shifted by 1000years, if the time when the older observer did their experiment was t, the time of the modern experiment is t +1000 years.
Split into simpler sentences: Recalling our two observers from different ages: the time in their experiments is shifted by 1000years. If the time when the older observer did their experiment was t, the time of the modern experiment is t +1000 years.