Complex sentence: The first verifiable date is that Turner was baptised on 14 May, and some authors doubt the 23 April date, on the grounds that high infant mortality rates would have meant that parents would usually baptise their children shortly after birth.
Split into simpler sentences: The first verifiable date is that Turner was baptised on 14 May. This casts some doubt on 23 April date, as high infant mortality rates meant parents would usually baptise their children shortly after birth.