It's worth noting that the reason the calends had slipped so badly is because it was Julius Caesar's job as pontifex maximus to handle the periodic updates to the calends that kept it in sync with the solar year. But he was too busy fighting in Gaul and having his adventures to bother with it for years, so the calendar slipped into chaos due to his mismanagement. He then went on to fix his screw-up, but he didn't just decide to fix the calendar, he was in a position where he'd screwed up so long he had to fix it.