Database Normal Form (financial statement)
My team once had to build a database for reporting financials with alternative time windows, including fiscal years, fiscal quarters, and fiscal months.  The relationship between dates and these units was documented, but really messy.  So here is what we did.

We created a table, call it Almanac, with one row per date.  (In reality, we had one row per work shift, but I'm leaving out that complexity for the sake of clarity).  We wrote an Almanac generating program that applied the strange rules for fiscal years, quarters and months to each date over a ten year period.  The PK for the Almanac table was the Date, although we couldn't use the name Date because it was a keyword.  The other columns were items like the fiscal year, the fiscal quarter, and flags like "Company Holiday". Notice that this program had one output, but no inputs.

Once the almanac was built, joining any other table with the almanac over a date field made it trivial to slice and dice by fiscal time periods, regardless of how obscure the rules were.  This worked like a charm.

(If you must know, the Almanac generator was written in COBOL+SQL.  This was a long, long, time ago, when Oracle was very young.)