DateTime - DateTime = TimeSpan
TimeSpan的属性里这么多:
public long Ticks { get; }
public int Days { get; }
public int Hours { get; }
public int Milliseconds { get; }
public int Minutes { get; }
public int Seconds { get; }
public double TotalDays { get; }
public double TotalHours { get; }
public double TotalMilliseconds { get; }
public double TotalMinutes { get; }
public double TotalSeconds { get; }
....