I can share some insights about setting fiscal year in QuickBooks Online.
You have the option to change the first month of the fiscal year depending on how you run your business. However, setting it to the last day of the month isn't possible. The start date of a fiscal year should begin on the 1st day.
To change the month on the fiscal year:
Click on the gear icon in the top right corner of QuickBooks Online.
Select Company & Account Settings.
Select Advanced, then select the pencil icon under Accounting.
Select the month for the First month of the fiscal year and the First month of the income tax year.
After you have chosen your new settings, select Save.
You can now click Done in the bottom right corner of your screen.
Additionally, here are some great references that you can check out about using QuickBooks Online: