The original Red Book was completed in III 3021, when Frodo Baggins finished adding his account of the War of the Ring, and so this is the earliest possible date for the Thain's Book to have been created. However, the Thain who requested its creation was presumably Peregrin Took, who did not accede to the title until IV 13, so the Thain's Book was probably produced after that date. It was certainly in existence by IV 64, when ThainPeregrin took it with him as he retired to Gondor.