(Approximate) Chronology of Horace's work
Satires ("Sermones"), Book 1 - 35-34 BC
Satires, Book 2 - 30-29
Epodes 30-29
Odes, Books1-3: 23 BC
Epistles, Book 1: 20-19
Odes, Book 4: 13
Epistles, Book 2: 13
but the Ars Poetica, "Art of Poetry" (Epistle 2.3): between 23 and 17