Valens began the thirteenth year of his reign in the March of 376, and this fact is one of Maran’s reasons for placing this letter where he does.  Tillemont reckons the thirteen years from 361 to 374, but Maran points out that if the Easterns had wanted to include the persecution of Constantius they might have gone farther back, while even then the lull under Julian would have broken the continuity of the attack.  Vit. Bas. xxxv.  cf. note on p. 48.