How do I truncate a number value in a field?


Can anyone help me figure this out?

I have a field I need to truncate the value of the sum of several other fields, dropping off the sum to make it a whole number. Now, when the other fields are null, it is blank. Perfect, life is good. But it rounds up, and I don't need or want it to round up. I need it to round off or down. I am not sure how to make it do that.

In the Modify Field I have:
AFNumber_Keystroke(0, 0, 0, 0, "", false);

In the Calculate Field I have:
AFSimple_Calculate("SUM", new Array("field1", "field2", "field3"))

In the Format Field I have:
if(event.value == 0) {
event.value = "";
AFNumber_Format(0, 0, 0, 0, "", false);
} else {
event.value = event.value;
AFNumber_Format(0, 0, 0, 0, "", false);