Performing date calculations using a calendar

First off I'd like to mention I'm a student taking C# this semester. The instructor I have has not been helpfull in teaching the language I feel, and I leave the class more confused than when I entered. She gave us an assignment that requires us to make a calendar that performs calculations with dates counting days to and past Valentines day which is then displayed in a richTextBox as the appropriate sentence depending on what date is selected within the monthCalendar. I am having problems with figuring out how to do the calculations and then displaying the number from the calculations within the sentence. I welcome any help and thank you in advance. The code i have from the .cs file is as follows:

using System;

using System.Collections.Generic;

using System.ComponentModel;

using System.Data;

using System.Drawing;

using System.Linq;

using System.Text;

using System.Windows.Forms;

namespace WindowsFormsApplication1

         public partial class Form1 : Form

                  public Form1()

                  private void Form1_Load(object sender, EventArgs e)

                  private void monthCalendar1_DateChanged(object sender, DateRangeEventArgs e)

                           if (e.Start.CompareTo(new DateTime(2009, 2, 14)) == 0)

                                    richTextBox1.Text = "Happy Valentines day";

                           else if (e.Start.CompareTo(new DateTime(2009, 2, 14)) <= 0)

                                    richTextBox1.Text = "Its #ofdays until Valentines Day";

                           else

                                    richTextBox1.Text = "Its been #ofDays since Valentines Day";