Marc,
I don't know what's going on here. My suggestion to you is to try and do this without using @Formulas. Find a pure JavaScript solution.
I'm not suggesting that the @Formulas should not work, but I am suggesting that in the long run it will be so much better for you to try and avoid using them as much as possible.
I don't know how to do this on JavaScript myself. But a quick Google let me to this question on StackOverFlow:
http://stackoverflow.com/questions/8516024/javascript-function-return-date-array-for-all-sundays-in-a-year
Which sounds like a pretty good start to what you're looking for.
Just my 2 cents.
Dave