The Gregorian calendar was adopted. Pope Gregory XIII introduced this calendar reform to correct inaccuracies in the Julian calendar. Roger Bacon proposed a calendrical reform similar to the later system introduced in 1582 under Pope Gregory XIII. Drawing on ancient Greek and medieval Islamic astronomy recently introduced to western Europe via Spain, Bacon continued the work of Robert Grosseteste and criticised the then-current Julian calendar as "intolerable, horrible, and laughable".

Pope Clement IV sends a reply to Roger Bacon's correspondence, commissioning "writings and remedies for current conditions". The Pope instructs Bacon not to violate any standing "prohibitions" of his order but to carry out his task in utmost secrecy. This papal patronage allowed Bacon to engage in a wide-ranging consideration of the state of knowledge in his era.