Time elapse computation in milliseconds C#

I need to time the execution of a code sequence written in C#. Using DateTime.Now I get incorrect values for the millisecond field. For example:

 int start_time, elapsed_time;

 start_time = DateTime.Now.Millisecond;

 for(int i = 0; i < N_ITER; i++) {
       // cpu intensive sequence
 }

 elapsed_time = DateTime.Now.Millisecond - start_time;

elapsed_time gives negative values.

How may I replace DateTime in order to obtain the actual value of the elapsed time?