The calendar stability that we are familiary with today does not go back indefinitely into the past. Before 5 AD it was very unstable. Use of historical dates from before this point has problems.
Most people alive today understand that that modern Gregorian calendar replaced the Julian. They also know the that the differences include the epoch, and the leap day rules. Most people typically don’t know that the year 5 AD is the first year when the Julian calendar was "stable" by our modern sense of the word. Starting in that year, and with all successive years until the adoption of the Gregorian calendar, the Roman world had a stable calendar following the rules of the Julian calendar.
Before 5 AD, the calendar was unstable with respect to the leap day rules. This instability sprung from an apparently accidental start of the calendar were leap days were incorrectly inserted every 3 years instead of every 4. When this error was caught, years with leaps were skipped until the extra leap days were worked out of the calendar. The years 4 AD, 1 BC and 5 BC should have been leap years, but they were not. These three missing leap years made up for the previous error.
This leap period covers the time of the Birth of Jesus so anyone trying to accurately determine the day of Jesus’ birth must be careful to use a calendar that reflects what actually happened in that time. We cannot count days forward from there, and determine things like anniversaries of that date without knowing and using the leap day rules for the era. The Life section is built with the correct calendar since it relies on these tools which are day-accurate back through the year 46 BC.
The year 46 BC was known as the "year of confusion" since it was exceptionally long. Looking backwards across time from the present, this is the earliest year which we can accurately choose individual days from dates given in historical sources. Before this year the leap process involved inserting of entire months like the Jewish calendar. Insertion of months in and of itself does not interfere with picking days from dates, but in the Roman world before 46 BC it does.
Unlike the Jewish calendar, which relied on specific rules for calendar structure, the Roman world used a political process when determining the insertion of a leap month. It required an act of the Roman Senate to insert such a month. This was not always done, when it "should have been" as we would think of it now. It was a political process subject to the same political whims we are so familiar with today, so exact placement of dates on a day-accurate scale is not possible, since the records of that era have not been passed down to us. All we can do now, for days in that period, is suggest the days for certain dates, which is what we do with the tools on this site.
Be aware that when a modern source gives a certain ancient date, usually associated with an astronomical event they are usually giving it on the modern Gregorian Calendar, or on an idealized Julian calendar, which, as we’ve already discussed, is only stable back to the year 5 AD. Don’t let someone fool you into thinking that this sort of date reflects what anyone in the world of the day actually lived. It is an artful idealization of the past.