Calculating DateTime in .NET from Unix timestamp

This article will describe how to calculate datetime in .NET . A Unix timestamp is defined as : - The number of SECONDS that has passed SINCE Unix Epoch, a defined date set to January 1st, 1970 at UTC (Universal Time Coordinated). We can calculate a Unix timestamp for now time if we call the following method:

	DateTimeOffset.UtcNow.ToUnixTimeSeconds()

For example, as of 10th August 2024 at 21:41 this method gives: 1723318757 To calculate the DateTime value from a Unix Timestamp we can define the following extension method, since we will have an interesting issue in 2038 on 19th January 2038 at 03:14:08 UTC. At that time, an overflow will occur for systems using 32-bits integers, which is actually quite a lot of systems included 32-bits time stamps in database systems. A solution of this is to switch to 64-bits long, but this is as stated not an issue until 2038, which is still about 14 years into the future as of 2024.. We will create extension methods for both seconds and milliseconds because sometimes Unix timestamps are given in milliseconds, and we should also support both int and long. In the future, that is 2038, int cannot be used, long must be used. In addition, extension method on string is also added for convenience.

public static class DateTimeExtensions {

	public static DateTime? FromUnixTimeSeconds(this string unixTimestamp) =>
		long.TryParse(unixTimestamp, out long unixTimestampCasted) ? 
			(DateTime?) CalculateDateTimeFromUnixTimeStamp(unixTimestampCasted) : null;

	public static DateTime FromUnixTimeSeconds(this int unixTimestamp) =>
		CalculateDateTimeFromUnixTimeStamp(unixTimestamp);

	public static DateTime FromUnixTimeSeconds(this long unixTimestamp) =>
		CalculateDateTimeFromUnixTimeStamp(unixTimestamp);
		
	private static DateTime CalculateDateTimeFromUnixTimeStamp(long unixTimeStamp) =>
		new DateTime(1970, 1, 1, 0, 0, 0, 0, DateTimeKind.Utc).AddSeconds(unixTimeStamp);

}

Here is how the method could be used in some examples:

    DateTime? dt = "1723318757".FromUnixTimeSeconds();
    DateTime? dt2 = DateTimeOffset.UtcNow.ToUnixTimeSeconds().ToString().FromUnixTimeSeconds();
	
	Console.WriteLine(dt.Value);
	Console.WriteLine(dt.Value.Kind);
	Console.WriteLine(dt2);
    

The screenshot below shows how it can be calculated.

Let's next look at an extension method that converts a DateTime to Unix time stamp.

    public static long ToUnixTimeSeconds(this DateTime dt) =>
		CalculateUnixTimeSeconds(dt);

	public static long? ToUnixTimeSeconds(this DateTime? dt) =>
		dt.HasValue ? CalculateUnixTimeSeconds(dt.Value) : null;

	private static long CalculateUnixTimeSeconds(this DateTime dt) =>
		(long) dt.ToUniversalTime().Subtract(DateTime.UnixEpoch).TotalSeconds;

Usage example:

DateTime? dt4 = DateTime.UtcNow;
Console.WriteLine(dt4.ToUnixTimeSeconds());    

This gives the unix time stamp from current time. Please note that it also possible to cast a DateTime into a DateTimeOffset and then just use built in method ToUnitTimeSeconds

long unixTimestamp5 = ((DateTimeOffset)(dt4)).ToUnixTimeSeconds();
Console.WriteLine(unixTimestamp5);
    

In addition to the methods above, adding support for milliseconds should be fairly straightforward. Officially, Unix time stamps are measured in seconds. And when you convert from a Unix timestamp to a datetime, you will get UTC time. To get the local time you can do this:

DateTime? dt = "1723325214".FromUnixTimeSeconds();
Console.WriteLine(dt.Value.ToLocalTime()); 
    

Use the method ToLocalTime() on the datetime of Kind Utc, i.e. convert from UTC datetime to local datetime, taking your time zone into account. The entire extension method class then looks like this:

DateTimeExtensions.cs

public static class DateTimeExtensions {

	public static long ToUnixTimeSeconds(this DateTime dt) => 
		CalculateUnixTimeSeconds(dt);

	public static long? ToUnixTimeSeconds(this DateTime? dt) =>
		dt.HasValue ? CalculateUnixTimeSeconds(dt.Value) : null;

	private static long CalculateUnixTimeSeconds(this DateTime dt) =>
		(long) dt.ToUniversalTime().Subtract(DateTime.UnixEpoch).TotalSeconds;

	public static DateTime? FromUnixTimeSeconds(this string unixTimestamp) =>
		long.TryParse(unixTimestamp, out long unixTimestampCasted) ? 
			(DateTime?) CalculateDateTimeFromUnixTimeStamp(unixTimestampCasted) : null;

	public static DateTime FromUnixTimeSeconds(this int unixTimestamp) =>
		CalculateDateTimeFromUnixTimeStamp(unixTimestamp);

	public static DateTime FromUnixTimeSeconds(this long unixTimestamp) =>
		CalculateDateTimeFromUnixTimeStamp(unixTimestamp);
		
	private static DateTime CalculateDateTimeFromUnixTimeStamp(long unixTimeStamp) =>
		new DateTime(1970, 1, 1, 0, 0, 0, 0, DateTimeKind.Utc).AddSeconds(unixTimeStamp);

}