C# - Measure Time Taken By A Function And Don'T Understand Why Time Difference Is Negative

I am trying to check how much time does a function takes to execute in my code :

I did this :

void foo()
{
int time = System.DateTime.Now.Millisecond;
// code of my function. do this, that, etc.
Console.WriteLine((System.DateTime.Now.Millisecond-time).ToString());
}

What i get is this :

-247

I don't understand why minus ?? & If pro-filer is an option then recommend me some simple option.

Note : Here I have shown a console app but in practice i need for a Web App in ASP.NET which may be n-tier architecture also.