Original:  The winter of 214/215 he spent at Nicomedia. Before 4 April 214 he had left Nicomedia, and in the summer he was at Antioch on the Orontes.
Revised:  The winter of 214/215 in Nicomedia. By 4 April 214 he had left Nicomedia, and in the summer he was at Antioch on the Orontes.
Edit intent: clarity