C# で UnixTime

var dt = DateTime.UtcNow;
//var dto = new DateTimeOffset(dt, new TimeSpan(+09, 00, 00));
var dto = new DateTimeOffset(dt);
long expires = dto.ToUnixTimeSeconds();

ネタ元

• https://webbibouroku.com/Blog/Article/unixtime-cs#outline__1
• https://www.sejuku.net/blog/51208#DateTimeNow