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Cyotek Historical Date Library

A screenshot of the demonstration application

Some months ago I was trying to create a timeline of British pre-history and needed to be able to store dates. The .NET DateTime or DateOnly structures are completely unsuitable for these as their ranges are far too small, nor do they allow for partial dates.

I did spent a little time poking around to see if there was some existing code, but there doesn't seem to be a lot of detail out there. I found a malformed blog post which was missing the bulk of the source code, and HistoricalDate class which was more fully fleshed, but still not fully suitable. So as typical, I went my own way and wrote my own.

Getting the library

The easiest way of obtaining the library is via NuGet.

Install-Package Cyotek.HistoricalDate

If you don't use NuGet, pre-compiled binaries can be obtained from the GitHub Releases page.

Of course, you can always grab the source and build it yourself!

About this library

The library currently offers two read-only structs, JulianDate and HistoricalTimeSpan.


The JulianDate structure can represent a partial date between 2147483647 BP and 2147483647 AD. By partial, I mean that a year and the era are always required, but the month and/or day is optional. After all, a date such as the Battle of Hastings may be documented but the start of the Mesolithic is a little more nebulous!


The HistoricalTimeSpan is a cut down version of the more familiar TimeSpan and is currently mainly used by the Add and Subtract methods of a JulianDate instance.

Using the library

Constructing instances

There are several constructors for specifying fully qualified or partial dates.

JulianDate(int year); // assumes AD
JulianDate(int year, JulianEra era);
JulianDate(int year, int month);  // assumes AD
JulianDate(int year, int month, JulianEra era);
JulianDate(int year, int month, int day);  // assumes AD
JulianDate(int year, int month, int day, JulianEra era);

// fully known
var fk = new JulianDate(2021, 9, 5); // September 5th, 2021 AD

// partial
var pd = new JulianDate(13000, JulianEra.Bc); // 130,000 BC

You can also use an explicit operator to convert the date portion of a DateTime instance into a JulianDate.

var now = (JulianDate)DateTime.UtcNow;

Parsing strings

You can also try and parse a string into a JulianDate instance.

Note: String parsing (and formatting) is somewhat basic (and potentially confusing) and will be improved in future updates to the library. Except with regards to month names, parsing is not culture-aware.

static JulianDate Parse(string s);
static bool TryParse(string s, out JulianDate result)

// fully known
var fk = JulianDate.Parse("2021-09-17"); // September 17, 2021 AD

// partial
var pd = JulianDate.Parse("40000 BP"); // 40,000 BC

Partial Dates

JulianDate supports partial dates, where only part of a date is known. The year and era are always required, but month and day are optional.

If day is specified, then the month is also available. The HasDay and HasMonth properties allow you to query what partial components are set, or if you just want to know if a date is fully known or partial, the IsFullyKnown and IsPartial properties can be used.

Accessing the Month or Day properties will return 0 if the component has not been set.

var pd = JulianDate.Parse("40000 BP");

pd.HasMonth; // false
pd.HasDay; // false

var pd = JulianDate.Parse("09 2021");

pd.HasMonth; // true
pd.HasDay; // false

Leap Years

The static IsLeapYear method will return if a given year is a leap year. This is calculated according to Scaliger.

JulianDate.IsLeapYear(42, JulianEra.Bc); // true
JulianDate.IsLeapYear(2021, JulianEra.Ad); // false

Things to improve

Currently, I'm unhappy with the string parsing as the list of accepted formats is too vague, and I haven't yet built in culture support.

The Subtract method could have serious performance issues when used with massive AD dates.


.NET Framework 2.0 or later.

Pre-built binaries are available via a signed NuGet package containing the following targets.

  • .NET 3.5
  • .NET 4.0
  • .NET 4.5.2
  • .NET 4.6.2
  • .NET 4.7.2
  • .NET 4.8
  • .NET 5.0
  • .NET Standard 2.0
  • .NET Standard 2.1
  • .NET Core 2.1
  • .NET Core 3.1

Is there a target not on this list you'd like to see? Raise an issue, or even better, a pull request.


About The Author


The founder of Cyotek, Richard enjoys creating new blog content for the site. Much more though, he likes to develop programs, and can often found writing reams of code. A long term gamer, he has aspirations in one day creating an epic video game. Until that time, he is mostly content with adding new bugs to WebCopy and the other Cyotek products.

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