Javascript stops working when form is viewed in Reader

I created a form in Acrobat 9.0 Pro that when you enter a date in Cell A, it auto-populates an amount depending on the date (using ">=" and "<=") in Cell B. At first I was having an issue getting it to work when i e-mailed it to a co-worker who had Acrobat 7.0. I had to do a CNTRL K and go into the JavaScript Preferences and check "Enable JavaScript debugger after acrobat is restarted" and check "Enable interactive console". After I did that, for some reason the form worked great. So now it works great in full versions of Acrobat Pro, but when opened and viewed in Reader 7.0, the Javascript stops working. No matter what date is entered, it auto-fills the highest amount, .95.
Below is my JavaScript.

var c = this.getField("MoYrPd1");
var t = this.getField("Rate1");

t.delay = true;
if(c.value=="")
t.value="";
else if(c.value<="04/10")
t.value = .75;
else if(c.value>="05/10")
t.value = .95;
t.delay = false;

Anybody have any ideas why it won't work in Reader?