lol, good point. Actually I just figured it out I think: I used =Year(STARTOFYEAR('Date Table'[date],"09/30")). This worked perfectly. But then I also wanted to have a new month column that had October as Month 1 and September as Month 12. This was more tricky and after some searching and trying formulas, I believe that it cannot be done.I don't think so, because there is no date Sept 31.
Without kidding, show us the file (or format of your data).
It's good practice to use a separate calendar table in your models but in this case it's basically obligatory!! There's a bunch of good resources at PowerPivotPro under the time intellgience tag: Time Intelligence « PowerPivotPro
Long story short, this calendar table has one row per date and is related to your fact table. The calendar table can contain as many columns as you like, one of which can be 'Fiscal Year'.