Complex sentence: Ever since the reforms under Ardashir I there had been no intercalation and so with a quarter day being lost each year the Zoroastrian holy year had slowly slipped backwards, with Nowruz eventually ending up in July.
Split into simpler sentences: Ever since the reforms under Ardashir I there had been no intercalation. Thus with a quarter day being lost each year, the Zoroastrian holy year had slowly slipped backwards, with Nowruz eventually ending up in July.