My understanding from reading the article is that The Definitive Biography of P. D. Q. Bach gives the DoB as 1 April 1742 and the DoD as 5 May 1807; it's just his grave where the dates are shown the other way round, followed by a question mark. On second thoughts, the bracketed form for DoB and DoD in the lead might lend itself to the reverse order plus a question mark. I admit that my removing the circular link to Baden-Baden-Baden was done from a "Wiki-instinct" and not appropriate for this page.
 the dates altogether from the lead seems rather dour and I suggest to reinstate this or a similar version:
P. D. Q. Bach (May 5, 1807, Baden-Baden-Baden –? April 1, 1742, Leipzig) …