Function to return the period associated with a date, when the period is 4-4-5 or 5-4-4

Has anyone written a complex expression that takes a date as an input, and determines the period associated with the date, when periods are not based on calendar ends (for example, 4-4-5, or 5-4-4)?

There doesn't appear to be a BRApi function for this, but perhaps I'm missing something. Absent a function, what techniques are recommended?

Thanks!