Reference for Rosh Hashanah 6:1

למנינא אחרינא הוא אמר רב פפא שנת עשרים שנת עשרים לגזירה שוה מה התם לארתחשסתא אף הכא לארתחשסתא

some other system of dating is adopted? - R'Papa replied: The occurrence in each text of the expression 'twentieth year' provides us with a gezerah shawah,<span class="x" onmousemove="('comment',' V. Glos.');">¹</span> [indicating that] just as in the latter case it means 'o the reign of Artaxerxes', so in the former. But how do you know that the incident of Kislev was prior?

Jastrow