Patient-year Incidence rates comparisons and p value

Dear all,

I would like to statistically compare two incidence rates from two different groupes.
I found the Incidence rate difference (IRD) formula from "Sahai and Kurshid, 1996".

The way to calculate (with examples) are presented in the following page :

My question is :
When the Chi-square related the IRD is calculated, how can I estimate the p value. In their example, Chi-square = 11.678635 P = .0006
How can I find .0006 from this Chi square. The number of degree of freedom is not detailed in the formula.

Thanks for your help!!