Genealogists often encounter challenges when looking up dates of events from many years ago. First of all, there's the Julian calendar versus the Gregorian calendar. Did you know that many people "lost" eleven days in 1792? Next, those researching Scandinavia ancestry often find dates recorded as falling on certain Feast Days with no reference to which day of the month it was. Anyone researching Jewish ancestry soon encounters issues with the Hebrew Calendar. Those with French ancestry may encounter the French Revolutionary Calendar. And then there are the Chinese (Lunar), Buddhist, Indian, Persian and even Aztec calendars.
The Ultimate Calendar Web Page can help you decipher dates. It can give you history about all of the above calendars and several more, including Baha'i and Moslem. It shows the phases of the moon and also provides a Gregorian to Julian date conversion utility.
Heck, this web site can even tell you what time it is.
The Ultimate Calendar Web Page.