Acho que consegui encontrar minha resposta - infelizmente isso não é possível.
Esta postagem no blog do MSDN do Excel MVP Zack Barresse diz:
Using timelines is as easy as point and click. When looking at the control you will see the filtered range colored, with each end containing a vertical ellipses, which you can click and drag to where you want (as seen below). In addition you can click any time segment seen in the control.
Your filtered data range will be shown in the Selection Label area. Too granular or course? Change the Time Level by clicking on it, where you get a drop down to choose from years, quarters, months and days. These are not customizable.
Confunde um pouco sobre o motivo de a Microsoft liberar essa função, sabendo que 35% de seus usuários principais nos EUA não usam o ano civil para o ano fiscal, incluindo a Microsoft! Esse número será ainda maior em outras partes do mundo. De wiki :
the fiscal year is identical to the calendar year for about 65% of publicly traded companies in the United States and for a majority of large corporations in the UK and elsewhere (with notable exceptions Australia, New Zealand and Japan).