David has a very good solution. A simpler expression is:
select year(dateadd(month, -3, start_date)) as FiscalYear
That is, subtract 3 months and take the year.
EDIT:
As noted in the comment, this seems to produce one year too early. Here are two solutions:
select year(dateadd(month, 9, start_date)) as FiscalYear
select 1 + year(dateadd(month, -3, start_date)) as FiscalYear
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