How can I convert a Unix timestamp to DateTime and vice versa?

Here's what you need:

public static DateTime UnixTimeStampToDateTime( double unixTimeStamp )
{
    // Unix timestamp is seconds past epoch
    DateTime dateTime = new DateTime(1970, 1, 1, 0, 0, 0, 0, DateTimeKind.Utc);
    dateTime = dateTime.AddSeconds( unixTimeStamp ).ToLocalTime();
    return dateTime;
}

Or, for Java (which is different because the timestamp is in milliseconds, not seconds):

public static DateTime JavaTimeStampToDateTime( double javaTimeStamp )
{
    // Java timestamp is milliseconds past epoch
    DateTime dateTime = new DateTime(1970, 1, 1, 0, 0, 0, 0, DateTimeKind.Utc);
    dateTime = dateTime.AddMilliseconds( javaTimeStamp ).ToLocalTime();
    return dateTime;
}

From Wikipedia:

UTC does not change with a change of seasons, but local time or civil time may change if a time zone jurisdiction observes daylight saving time (summer time). For example, local time on the east coast of the United States is five hours behind UTC during winter, but four hours behind while daylight saving is observed there.

So this is my code:

TimeSpan span = (DateTime.UtcNow - new DateTime(1970, 1, 1, 0, 0, 0, 0,DateTimeKind.Utc));
double unixTime = span.TotalSeconds;

You can use DateTimeOffset.

For example. I have a DateTime object

var dateTime1 = DateTime.Now;

If I want to convert it to the Unix time stamps, it can be achieved as follows

var unixTimeSeconds = new DateTimeOffset(dateTime1).ToUnixTimeSeconds()

If you want convert unix timeStamp to normal DateTime, you can use this code piece:

var dateTime2 = DateTimeOffset.FromUnixTimeSeconds(unixTimeSeconds).LocalDateTime;

or

var dateTime2 = DateTimeOffset.FromUnixTimeSeconds(unixTimeSeconds).UtcDateTime;

For more information please visit this link :

DateTimeOffset.ToUnixTimeSeconds Method,
DateTimeOffset.FromUnixTimeSeconds

Unix epoch is now part of DateTime class. Converting millis to DateTime is as easy as

DateTime.UnixEpoch.AddMilliseconds(millis)

In C# 8.0 (.NET Core 2.1) it is a simple one liner:

DateTime.UnixEpoch.AddSeconds(unixTimeInSeconds)

var dt = DateTime.Now; 
var unixTime = ((DateTimeOffset)dt).ToUnixTimeSeconds();

// 1510396991

var dt = DateTimeOffset.FromUnixTimeSeconds(1510396991);

// [11.11.2017 10:43:11 +00:00]

From .net 4.6, you can do this:

var dateTime = DateTimeOffset.FromUnixTimeSeconds(unixDateTime).DateTime;

Written a simplest extension that works for us. If anyone looks for it...

public static class DateTimeExtensions
{
    public static DateTime FromUnixTimeStampToDateTime(this string unixTimeStamp)
    {

        return DateTimeOffset.FromUnixTimeSeconds(long.Parse(unixTimeStamp)).UtcDateTime;
    }
}

System.DateTimeOffset.Now.ToUnixTimeSeconds()

A Unix tick is 1 second (if I remember well), and a .NET tick is 100 nanoseconds.

If you've been encountering problems with nanoseconds, you might want to try using AddTick(10000000 * value).

public static class UnixTime
    {
        private static readonly DateTime Epoch = new DateTime(1970, 1, 1, 0, 0, 0, 0);

        public static DateTime UnixTimeToDateTime(double unixTimeStamp)
        {
            return Epoch.AddSeconds(unixTimeStamp).ToUniversalTime();
        }
    }

you can call UnixTime.UnixTimeToDateTime(double datetime))