Complex sentence: The day and month indicated in the "Dates of reign" column indicates the first day of their reign period, which was also the first day of the lunisolar year following the death of the previous emperor.
Split into simpler sentences: The day and month that appear under "Dates of reign" indicate the first day of the era by which each emperor is known. This was also the first day of the lunisolar year following the death of the previous emperor.