Custom Calendar

Hi All

I am creating a custom calendar in the query Editor I want to add a column that if the week number is >=14 then minus13 else add 39. I think I have to add a custom column but I don't know how to write the formula in M lanuage if I am correct in saying. I have tried the condition column but I don't think I can perform this task using it. I should mention that I have a week number column

If you can guide me in the right direction I would appreciate it

Thanks for your time