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

The latest version of .NET (v4.6) has 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 UTC DateTimeOffset:

DateTimeOffset dateTimeOffset = DateTimeOffset.FromUnixTimeSeconds(1000);

• DateTimeOffset to Unix time in seconds:

long unixTimeStampInSeconds = dateTimeOffset.ToUnixTimeSeconds();

• Unix time in milliseconds to UTC 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 a UTC DateTimeOffset. To get a DateTime representation simply use the DateTimeOffset.UtcDateTime or DateTimeOffset.LocalDateTime properties:

DateTime dateTime = dateTimeOffset.UtcDateTime;

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;
}

DateTime to UNIX timestamp:

public static double DateTimeToUnixTimestamp(DateTime dateTime)
{
    return (TimeZoneInfo.ConvertTimeToUtc(dateTime) - 
           new DateTime(1970, 1, 1, 0, 0, 0, 0, System.DateTimeKind.Utc)).TotalSeconds;
}