since we had a calendar feature the other day, i thought i would also include this description of the roman calendar, found here, and perhaps also will soon post about the athenian calendar:
Prior to the reform (replacement is a better word) of the calendar by Julius Caesar, the Romans used a calendar which was made up of twelve familiarly-named months of varying lengths: Martius, Maius, Quintilis (later known as Julius), and October each had 31 days; Ianuarius, Aprilis, Iunius, Sextilis (later known as Augustus), September, November and December each had 29 days; Februarius had 28 days. That provided for a 355-day year which is, probably uncoincidentally, the length of a lunar year. Obviously it wouldn't take very long for such a calendar to get out of whack with reality, so it was the practice (apparently) to regularly 'intercalate' a month (i.e. insert a new month) of 22 (sometimes 23) days in alternating years after the festival known as Terminalia (Feb. 23). This month was referred to as Intercalaris and would have the last five days of February added onto it, resulting in a month of either 27 or 28 days. [Something I've never quite figured out: given the apparent importance of birthdays to the Romans (to judge by epigraphy), when they claimed to have lived 'x number of years, x number of months, x number of days', how long were the years?].
Unfortunately even with the provisions for intercalations, the calendar of the Roman Republic often went out of whack anyway and Julius Caesar decided to fix things once and for all (or so he thought). By virtue of his being pontifex maximus, he threw out the old calendar and replaced it with a calendar which had 365.25 days, which astronomers in Egypt and elsewhere had long known to be the actual length of a 'tropical year'. Caesar also fiddled with the number of days in the months to what we are used to, and made provisions for an extra day in February (February 24 happened twice; that would be handy if you had a term paper due, no?). Unfortunately, because the Romans counted inclusively and the folks who told Caesar about this calendar didn't, subsequent pontifices were adding a day every three years instead of every four, and so Augustus would later have to correct this.
The final thing to note about the Roman calendar is that they had a somewhat peculiar system of deciding the 'number' of the day. The first day of every month was known as Kalendae (the 'kalends'); the fifth day (or, in certain months, the seventh) was referred to as Nonae (the 'nones'), the thirteenth (or, in certain months, the fifteenth) was referred to as Idus (the 'Ides'). All other days were referred to by counting backwards from these fixed points in the month. And so, for example, while the Roman equivalent of January 1 would be Kalendae Ianuariae (often abbreviated Kal. Ian.), January 2 would be designated ante diem IV Nonas Januarias (IV Non. Ian). And since January is a day when the 'Nones' falls on the equivalent of the fifth, January 4 was designated pridie Nonas Ianuarias (pr Non. Ian).
The upshot of all this is that This Day in Ancient History provides the date equivalencies according to the calendar implemented by Julius Caesar, using the peculiar system of designating days described above. In some cases, this will be somewhat anachronistic, especially when providing day equivalencies for events which happened (or festivals celebrated) prior to this reform (e.g. there was no such thing as September 30 prior to Julius Caesar's calendar). As far as I'm aware, though, ceteris paribus, the days will only be one or two days off and I'm sure that won't offend the pax deorum.