How do you convert epoch time in C#?

You actually want to AddMilliseconds(milliseconds), not seconds. Adding seconds will give you an out of range exception.

The latest version of .Net (v4.6) just added built-in support for Unix time conversions. That includes both to and from Unix time represented by either seconds or milliseconds.

• Unix time in seconds to DateTimeOffset:

DateTimeOffset dateTimeOffset = DateTimeOffset.FromUnixTimeSeconds(1000);

• DateTimeOffset to Unix time in seconds:

long unixTimeStampInSeconds = dateTimeOffset.ToUnixTimeSeconds();

• Unix time in milliseconds to DateTimeOffset:

DateTimeOffset dateTimeOffset = DateTimeOffset.FromUnixTimeMilliseconds(1000000);

• DateTimeOffset to Unix time in milliseconds:

long unixTimeStampInMilliseconds= dateTimeOffset.ToUnixTimeMilliseconds();

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Note: These methods convert to and from DateTimeOffset. To get a DateTime representation simply use the DateTimeOffset.DateTime property:

DateTime dateTime = dateTimeOffset.UtcDateTime;

With all credit to LukeH, I've put together some extension methods for easy use:

public static DateTime FromUnixTime(this long unixTime)
{
    var epoch = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);
    return epoch.AddSeconds(unixTime);
}

public static long ToUnixTime(this DateTime date)
{
    var epoch = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);
    return Convert.ToInt64((date - epoch).TotalSeconds);
}

Note the comment below from CodesInChaos that the above FromUnixTime returns a DateTime with a Kind of Utc, which is fine, but the above ToUnixTime is much more suspect in that doesn't account for what kind of DateTime the given date is. To allow for date's Kind being either Utc or Local, use ToUniversalTime:

public static long ToUnixTime(this DateTime date)
{
    var epoch = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);
    return Convert.ToInt64((date.ToUniversalTime() - epoch).TotalSeconds);
}

ToUniversalTime will convert a Local (or Unspecified) DateTime to Utc.

if you dont want to create the epoch DateTime instance when moving from DateTime to epoch you can also do:

public static long ToUnixTime(this DateTime date)
{
    return (date.ToUniversalTime().Ticks - 621355968000000000) / 10000000;
}

UPDATE 2020

You can do this with DateTimeOffset

DateTimeOffset dateTimeOffset = DateTimeOffset.FromUnixTimeSeconds(epochSeconds);
DateTimeOffset dateTimeOffset2 = DateTimeOffset.FromUnixTimeMilliseconds(epochMilliseconds);

And if you need the DateTime object instead of DateTimeOffset, then you can call the DateTime property

DateTime dateTime = dateTimeOffset.DateTime;

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Original answer

I presume that you mean Unix time, which is defined as the number of seconds since midnight (UTC) on 1st January 1970.

private static readonly DateTime epoch = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);

public static DateTime FromUnixTime(long unixTime)
{
    return epoch.AddSeconds(unixTime);
}