The fact that we use the same month names as the Romans encourages us to assume that a Roman date occurred on the same Julian date as its modern equivalent. This assumption is not correct. Even early Julian dates, before the leap year cycle was stabilised, are not quite what they appear to be. For example, it is well known that Julius Caesar was assassinated on the Ides of March in 44 BC, and this is usually converted to 15 March 44 BC. While he was indeed assassinated on the 15th day of the Roman month Martius, the equivalent date on the modern Julian calendar is probably 14 March 44 BC.
I’ve noticed that historians put a lot of effort into figuring out what day things really happened. Was Caesar assassinated on the 15th or the 14th? Was Washington born on February 11th or February 22nd?
The thing is, I’ve always thought historians were barking up the wrong tree on this. To my way of thinking, it really doesn’t matter what day something really happened; what matters more is what day people thought it happened. So as far as I’m concerned, George was born on the 11th because his mother and father, the attending midwife, and the Smith family down the road all thought of the day as the 11th.
Here’s another example: say I was born on March 8th. Fifty years later a congressional proclamation changes the calendar slightly, and now I’m told I was born on March 4th. But I’ve thought I was born on March 8th all my life; I’ve said “March 8th” about ten thousand times, and I’m very familiar with how it sounds. March 8th fits me like an old shoe. Now you’re asking me to put this squeaky new shoe on, this March 4th shoe. Thanks, but no thanks. I wasn’t born on March 8th because that’s what a calendar said, not really. I was born on March 8th because that’s what we called the day I was born on when I was born on it.