As long as you file that extension, you are good until the extension deadline. Don't pull a Michael Scott and just "declare it".
https://www.youtube.com/watch?v=Zp4BQYV0-P0From the IRS site:
https://www.irs.gov/retirement-plans/retirement-plans-faqs-regarding-seps-contributionsDepositing and deducting contributions
When must I deposit the contributions into the SEP-IRAs?
You must deposit contributions for a year by the due date (including extensions) for filing your federal income tax return for the year. If you obtain an extension for filing your tax return, you have until the end of that extension period to deposit the contribution, regardless of when you actually file the return.
If you did not request an extension to file your tax return and did not deposit the SEP plan contributions by the filing due date for that return, you are not allowed to deduct any SEP plan contributions on that year’s return. The contributions may be deducted on the following year’s return.
If you improperly deducted SEP plan contributions on your return, you must file an amended tax return as soon as possible.