JSON Web Token - exp, nbf, iat claims - convert DateTime to Unix time

In JWT following claims must be a number containing a date as a number:

exp (Expiration Time) Claim
nbf (Not Before) Claim
iat (Issued At) Claim

If you look into IETF RFC7519 you can find information about what exactly is this number:

A JSON numeric value representing the number of seconds from 1970-01-01T00:00:00Z UTC until the specified UTC date/time, ignoring leap seconds. This is equivalent to the IEEE Std 1003.1, 2013 Edition [POSIX.1] definition “Seconds Since the Epoch”, in which each day is accounted for by exactly 86400 seconds, other than that non-integer values can be represented. See RFC 3339 RFC3339 for details regarding date/times in general and UTC in particular.

If you develop your application with .NET Framework 4.6 or higher you can use DateTimeOffset.ToUnixTimeSeconds method. But if your .NET Framework is lower you can create an extension method to convert DateTime to Unix timestamp. Below is simple implementation with such implementation:

public static class DateTimeExtensions
{
    public static long ToUnixTimestamp(this DateTime date)
    {
        var epoch = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);
        var time = date.ToUniversalTime().Subtract(epoch);
        return time.Ticks / TimeSpan.TicksPerSecond;
    }
}

If Unix timestamp is storing as a signed 32-bit binary integer in the year 2038 will be the problem known as “Year 2038 problem”.

Resources:

https://tools.ietf.org/html/rfc7519

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