c#, DateTime to UnixTime / UnixTime to DateTime Convert

public DateTime UnixTimeToDateTime(double value) {
    DateTime unixEpoch = new DateTime(1970, 1, 1, 0, 0, 0, 0).ToLocalTime();
    return unixEpoch.AddSeconds(value);
}
public double DateTimeToUnixTime(DateTime value) {
    TimeSpan span = (value - new DateTime(1970, 1, 1, 0, 0, 0, 0).ToLocalTime());
    return (double) span.TotalSeconds;
}