/// <summary>
 /// Add seconds
 /// </summary>
 /// <param name="i"></param>
 /// <param name="seconds"></param>
 /// <returns></returns>
 /// <exception cref="ArgumentNullException"></exception>
 public static Instant AddSeconds(this Instant i, double seconds) => i.PlusSeconds(seconds);
 /// <summary>
 /// Add seconds
 /// </summary>
 /// <param name="i"></param>
 /// <param name="seconds"></param>
 /// <returns></returns>
 /// <exception cref="ArgumentNullException"></exception>
 public static Instant AddSeconds(this Instant i, long seconds) => i.PlusSeconds(seconds);