Simple operation of date and time in C

(1). Compare the size of two DateTime

DateTime dt1 = Convert.ToDateTime("2010/11/25 20:53:43");
DateTime dt2 = Convert.ToDateTime("2010/11/26 19:24:53");
int i = DateTime.Compare(dt1, dt2);

And then finally i is -1... So dt1 < dt2.. When dt1=dt2 i=0.. When dt1 > dt2, i=1

(2). Calculate the difference between two DateTime

DateTime dt1 = Convert.ToDateTime("2010/11/25 20:53:43");
DateTime dt2 = Convert.ToDateTime("2010/11/25 20:53:43");
DateTime dt3 = Convert.ToDateTime("2010/11/25 20:53:43");
TimeSpan diff1 = dt2.Subtract(dt1);
DateTime diff2 = dt3.Subtract(dt1);
TimeSpan diff3 = dt2 - dt3;
DateTime diff4 = dt1 - dt2;

The result is:

diff1="185.14:47:00";(185 day 14 hours 47 minutes 0 seconds )
　　diff2="1996/4/9 17:55:00";
　　diff3="55.04:20:00";(55 day 4 hours 20 minutes 0 seconds )
　　diff4="1996/4/9 17:55:00";

(3). Calculate a date minus or plus the date after a certain number of days

TimeSpan ts = new TimeSpan(40, 0, 0, 0); 
DateTime dt1 = DateTime.Now.Subtract(ts); 
DateTime dt2 = DateTime.Now.Add(ts); 
string str1 = ts.Days.ToString() + " Days ago is " + dt1.ToString("yyyy years MM month dd day "); 
string str2 = ts.Days.ToString() + " Days is " + dt2.ToString("yyyy years MM month dd day ");

The result is:

str1="40 Days ago is 2010 years 10 month 16 day ";
str2="40 Days is 2011 years 01 month 04 day ";

PS:(calculated as 25 November 2010)