Philip SEISYLL (ap Thomas) - Date of Birth

With this ancestry trail, surely the date of birth of Philip cant be 1366 or else something else in the tree is wrong?

Margaret Hanmer Margaret Hanmer 1357
Margaret verch Owain Margaret ferch Owain 1380
Joan Monnington Joan Monnington 1380
Thomas Seisyll Thomas Seisyll of Altyrynys 1335
Philip Seisyll Philip Seisyll 1366
Richard Cecil Richard Cecil 1431