0DTE options: what expiry does to the premium, and assignment

What DTE means in options, and why the number is doing work

DTE is simply days to expiry. A contract with 45 DTE expires in forty-five days; one with 0 DTE expires today. The reason the number gets its own vocabulary is that it is not a label but an input: the extrinsic half of every premium is priced on how much time remains and how much the share price might move within it. Understanding what DTE means in options is therefore inseparable from understanding what you are paying for.

0DTE options are the boundary case of that input. The contract still obliges exactly what it always obliged, 100 shares at the strike, but the time component that made it an option rather than a bet has almost entirely elapsed. Everything unusual about the final day follows from that one fact, and nothing about it requires new rules.

What expiry does to the premium

A premium is intrinsic value plus extrinsic value. Intrinsic value is the part already earned: for a call, the share price above the strike. Extrinsic value is everything else, and it is compensation for the remaining time and uncertainty. As expiry approaches, extrinsic value falls, and it falls at an accelerating rate: the last day sheds far more of it proportionally than any earlier day, because there is proportionally far less possibility left to price.

By the final session the premium of a 0DTE contract is close to pure intrinsic value. Two dollars in the money means roughly two dollars of premium; out of the money means very little premium at all, and what remains can vanish within minutes. The practical effect is that the contract now tracks the share price almost one-for-one on the in-the-money side and barely responds on the other, so the smooth curve familiar from longer expiries has straightened into the hinge you see in the diagram.

Why assignment risk peaks at the close

Assignment is the writer being held to the obligation because the holder exercised. It is most likely precisely when there is no time value left to surrender, which is expiry day, so short positions in 0DTE options carry the highest assignment probability of any point in the contract's life. For a covered call that means the shares are called away at the strike. For a cash-secured put it means 100 shares arrive at the strike and the collateral is spent. Both are the contract working, and both are why the collateral existed.

There is one further wrinkle that catches writers repeatedly. Exercise decisions can be made in a window after the regular session closes, while the underlying still trades elsewhere. A short contract that finished the day narrowly out of the money can therefore still be assigned if the price moves afterwards. A writer who treats the closing price as final has misread when the obligation ends. Assignment ends it, not the bell.

Why a contract expires today at all

Daily expiries are a listing decision, not a market phenomenon. Exchanges added expiration dates to the most heavily traded index and single-name option series until, on some products, a contract expires on every trading day of the week. Nothing about the contract specification changed to allow it. A 0DTE contract is an ordinary option whose expiry happens to be today, and every rule in this section applies to it exactly as it applies to a contract with ninety days left.

That matters because the phrase is routinely used as though it named a strategy. It names a date. The reason the behavior looks unfamiliar is that the same mechanics produce very different proportions when the time input approaches zero: extrinsic value has almost nothing left to give up, so the position tracks the underlying almost directly, and gamma: the rate at which its directional exposure itself changes: reaches its maximum. Those are properties of the arithmetic at that boundary, not properties of a new instrument.

Settlement, and the difference that decides your risk

One distinction is worth more than everything else on this page for a seller: whether the contract settles in shares or in cash. Single-name equity options settle physically. If a short position finishes in the money, 100 shares per contract change hands, and the account has to be able to deliver or to receive them. A cash-settled index option resolves to a payment instead, so there is no delivery obligation and no unwanted position the next morning.

The consequence for 0DTE options is direct. A physically settled short contract carries assignment risk right through the close and into the window afterwards, and the outcome is a stock position you did not choose, financed at whatever the market opens at. A cash-settled one cannot do that; its worst case is a debit. Two positions with identical payoff diagrams therefore carry materially different overnight risk, and the diagram does not show it. Reading the contract specification before the final day, rather than reading the payoff, is what separates the two.

The arithmetic that the low price hides

Short-dated contracts look inexpensive because their absolute premium is small. Measured against what they actually buy, which is the days in which the move can happen, they are the most expensive part of the expiry curve. Buying a contract for a tenth of the premium of one three weeks out is not a discount if it carries a thirtieth of the time. Expiry is not a variable to be economized on; it is the quantity being bought.

That is the whole case for treating 0DTE options as a mechanism to understand rather than a strategy to adopt. The premium is nearly all intrinsic, so there is little cushion. Assignment is at its most probable, so a short position is at its most exposed. And the position's sensitivity to the share price is at its most extreme, which is a statement about gamma rather than about anyone's skill. All three follow from the same number reaching zero.