Properly calculating calendar dates from lower-level data manually is really annoying. You need to deal with time zones, leap days, daylight savings, leap seconds...
Our current Gregorian calendar is a slight modification of the Julian calendar, named after Julius Caesar himself who introduced it in 45BC. If he hadn't done that, and nobody afterwards had bothered to either, we'd still be stuck with the traditional Roman calendar. Here, I collect a few “interesting” issues we'd run into when attempting to use it for modern purposes.
You might have read somewhere that the Romans counted years from AUC — Ab Urbe Condita, “from the founding of the city”. Sadly it's not going to be that easy. By far, the common method of specifying what year you were talking about was to mention what consuls were serving during that time. So, for example, you'd have to say something like “During the consulship of L. Licinius Crassus and Q. Mucius Scaevola” to refer to 95 BC.
A consular term lasted for one year and you'd use the names of the consuls elected first — if a consul died and was replaced in the middle of the year, the name of the new consul was not used for dating.