Quite simple :
DateTime start = new DateTime(1, 1, 2013);
DateTime end = DateTime.Now;
TimeSpan ts = end - start;
int totalDays = ts.TotalDays;
Hope this helps.
[Edit] Sorry I didn't make attention to the tag. Here's in VB.NET :
Dim startDate As DateTime = New DateTime(1, 1, 2013)
Dim endDate As DateTime = DateTime.Now
Dim ts As TimeSpan = endDate - startDate
Dim totalDays As Integer = ts.TotalDays