декомпиляция DEX в исходный код Java

Если вы не используете 4.6, это может помочь Источник: System.IdentityModel.Tokens

    /// 

    /// DateTime as UTV for UnixEpoch
    /// 
    public static readonly DateTime UnixEpoch = new DateTime(1970, 1, 1, 0, 0, 0, 0, DateTimeKind.Utc);

    /// 

    /// Per JWT spec:
    /// Gets the number of seconds from 1970-01-01T0:0:0Z as measured in UTC until the desired date/time.
    /// 
    /// The DateTime to convert to seconds.
    /// if dateTimeUtc less than UnixEpoch, return 0
    /// the number of seconds since Unix Epoch.
    public static long GetIntDate(DateTime datetime)
    {
        DateTime dateTimeUtc = datetime;
        if (datetime.Kind != DateTimeKind.Utc)
        {
            dateTimeUtc = datetime.ToUniversalTime();
        }

        if (dateTimeUtc.ToUniversalTime() <= UnixEpoch)
        {
            return 0;
        }

        return (long)(dateTimeUtc - UnixEpoch).TotalSeconds;
    }