If you’re a C corporation, the IRS lets you choose between the standard calendar year or your own fiscal year schedule.
If you’re a partnership or an S corporation, you need to get permission from the IRS before adopting a fiscal year.
you needed to file form 8716
https://www.irs.gov/pub/irs-pdf/f8716.pdf
a single member LLC is a disregarded entity so would be treated as a sole proprietorship that must use a calendar year.
