Updated July 28, 2026. Quick answer: Thirty days from the date of transfer, with no extension available. An 83(b) election taxes you on the spread now — usually near zero on an early exercise — and starts your capital gains clock immediately.
What the election does
Without it, unvested shares you have purchased are taxed as they vest, at whatever the value is then. With it, you elect to be taxed on the whole spread at transfer. Exercise early enough and that spread is close to zero, so you pay almost nothing and every subsequent dollar is capital gain.
The deadline is absolute
Thirty calendar days from transfer. It is not extended by weekends, holidays, or a filing extension on your return, and there is no general relief for missing it. IRC §83(b)(2).
The risk nobody prices
If you pay tax on the spread and the shares later become worthless, or you leave before vesting and forfeit them, you generally do not get that tax back. The election is a bet that the shares will be worth something. On an early exercise at a near-zero spread the bet costs little; later, when the spread is large, it can be a substantial and irreversible payment.
Practical filing notes
File with the IRS office where you file your return, keep proof of mailing, and give a copy to your company. The proof matters — the burden of showing timely filing is yours.
Sources
IRC §83(a), (b); IRC §83(b)(2) (30-day window); Treas. Reg. §1.83-2.
This states what the cited authority says. It is not tax advice; AMT in particular is computed across your whole return and cannot be resolved from one page.