Complex sentence: The date of his reign is unknown, as the date of the conversion is hotly disputed, though it is certain that Bulan reigned some time between the mid 700's and the mid 800's.
Split into simpler sentences: The date of his reign is unknown, as the date of the conversion is hotly disputed. However, it is certain that Bulan reigned some time between the mid 700's and the mid 800's.