How can I convert a DateTime to the number of seconds since 1970?

The only thing I see is that it's supposed to be since Midnight Jan 1, 1970 UTC

TimeSpan span= DateTime.Now.Subtract(new DateTime(1970,1,1,0,0,0, DateTimeKind.Utc));
return span.TotalSeconds;

If the rest of your system is OK with DateTimeOffset instead of DateTime, there's a really convenient feature:

long unixSeconds = DateTimeOffset.Now.ToUnixTimeSeconds();

You probably want to use DateTime.UtcNow to avoid timezone issue

TimeSpan span= DateTime.UtcNow.Subtract(new DateTime(1970,1,1,0,0,0)); 

That's basically it. These are the methods I use to convert to and from Unix epoch time:

public static DateTime ConvertFromUnixTimestamp(double timestamp)
{
    DateTime origin = new DateTime(1970, 1, 1, 0, 0, 0, 0, DateTimeKind.Utc);
    return origin.AddSeconds(timestamp);
}

public static double ConvertToUnixTimestamp(DateTime date)
{
    DateTime origin = new DateTime(1970, 1, 1, 0, 0, 0, 0, DateTimeKind.Utc);
    TimeSpan diff = date.ToUniversalTime() - origin;
    return Math.Floor(diff.TotalSeconds);
}

Update: As of .Net Core 2.1 and .Net Standard 2.1 a DateTime equal to the Unix Epoch can be obtained from the static DateTime.UnixEpoch.