I need to represent dates earlier than January 1 0001. Do you know a library that includes such a type?
It should provide the following functionality:
It would be ok if values would be less exact the longer the date is ago.
Take a look at this thread on SO with exactly the same question. Basically, there is no built-in support, so you'll need to find a 3rd party implementation or build it yourself.
If you really only need those three operations (meaning no day-of-week, day-of-month, time zones etc): just use long as number of seconds (or milliseconds) from pivot date. 2^64 seconds give you half a trillion of years.
There were 445 days in 45 BC, so you'll need to add that in, and the switch from Julian (45bc) to the previous roman calendar. The Gregorian calendar change came a lot later (1500s) than you need so you don't have to worry about that.
The two main problems of using DateTime are the min and max constants. Obviously being a struct you can't subclass it either.
My first option would be to write my BigDateTime or BCDateTime struct, and use a DateTime inside that via composition. You can then set your current Calendar:
CultureInfo.CurrentCulture.DateTimeFormat.Calendar = new JulianCalendar();
for to the Julian one but as I mentioned that only gives you to 45 years BC
If the year is below 45 you switch Calendars again to your own subclassed Calendar object. If the year is 45, you'll need another Calendar with the 445 days for that year (they did this to adjust between the older Calendar).
What will your min and max constants be for this object? The dawning of civilisation itself?!
You will need to pick those, and also bare in mind the Julian calendar and its previous non-reformed version were specific for the countries of the Roman empire. Being a westerner I have no idea of the other calendars that existed, you'll get a lot more information from the wikipedia articles though.
If you choose to do all cultures then you have a mammoth task on your hands (as the original .NET team did) to implement calendars that already exist. Different calendars have different starting epochs relative to BC, such as the Islamic, Chinese and Hindu calendars. More information on that is here. So if you want every culture and not just the Roman empire you will need to make your own version of the following, or atleast some of them:
System.Globalization.Calendar
System.Globalization.EastAsianLunisolarCalendar
System.Globalization.GregorianCalendar
System.Globalization.HebrewCalendar
System.Globalization.HijriCalendar
System.Globalization.JapaneseCalendar
System.Globalization.JulianCalendar
System.Globalization.KoreanCalendar
System.Globalization.PersianCalendar
System.Globalization.TaiwanCalendar
System.Globalization.ThaiBuddhistCalendar
System.Globalization.UmAlQuraCalendar
Good luck! It'd be interesting to see how you get on.