by having a 445-days-long year in 46bc. [en.wikipedia.org/wiki/julian\ this means that the romans started "fresh" in 45bc and just needed to follow the 1-leap-day-every-4-years system to be in sync with nature - and they screwed it up. the switch from julian to gregorian happened 1600 years later, and it was necessary because the julian calendar is actually actually slightly faster than nature (the earth's rotation around