减去VBA中的日期?

I'm having big problems doing operation with the date in Excel VBA. I have a form that has a textbox where the user will enter the date. The problem is that he may enter it in different formats (eg, 1.08.2011 for 1st of August, or 8/1/11 for the same day). Now what I want to do is to subtract some days from that date that he enters in the TextBox. I had to success so far and I don't know how to do it. I tried something like thisI'm having big problems doing operation with th