Period count when year starts

[Bart_Breekveldt]

Dear community,

I've made a new date function, a period of four weeks (we don't use months, but periods which results in 13 instead of 12 date periods per year). The script which I've used to make the field in the pivot table is ("Datum-Tijd" = Date-Time in English)

=Dual('P'&Num(Week([Datum-Tijd]/4),00),Num(Week([Datum-Tijd]/4),00))

(The installed sensors, which count the passing trays, are brand new and in the test phase, so not four weeks are shown per period)

Here is my question: how can I set the period count to start when the year starts? So for example week 1-4 are period 1 and week 5-8 period 2. The function gives me periods in de 40s, which is unhelpful. 

Do you know how to start the period count when the year starts?

Thanks in advance.