[rules-users] Best Practices - using functions

So I'm sitting here with a need to get the day of year on a date field, and wondering if it doesn't make sense to have this on the original object. The rule would be something simple, like when thisdayofyear < thatdayofyear then dosomething() Now, if the day of year is computed on the fly with a helper function, it would seem this would always be put on the agenda as the inference engine would not know the computed value in time. Does this make sense? Robert Costello