The short answer
The difference between a 0DTE and a 1DTE option is one overnight gap, and that single gap changes three separate things.
- The Greeks are smaller on the 1DTE, by a known factor. Gamma and theta both scale with the inverse square root of time remaining, so the same-day contract carries the larger figure per point of underlying move.
- The 1DTE carries exposure through a period it cannot be managed across. The underlying's futures keep trading; the option's own market is thinner or shut for most of that window.
- They are visible in positioning data on completely different terms. A 1DTE opened today lands in tomorrow's settled open interest. A 0DTE opened and closed today lands nowhere at all.
How same-day options behave covers 0DTE contracts on their own terms, and 0DTE gamma exposure covers the levels computed from the front expiration alone. This page is only about what the extra night does.
The arithmetic on the Greeks
At-the-money gamma scales roughly as one over the square root of time to expiry. That makes the comparison concrete rather than directional.
Take an ordinary session with six and a half hours of cash trading left in it, against a contract expiring at the following session's close, roughly thirty and a half hours away in calendar terms. The square root of thirty and a half over six and a half is about 2.2, so the same-day at-the-money contract carries around 2.2 times the gamma of the next-day one, holding spot and implied volatility equal. Theta per day follows the same rule for an at-the-money contract, which is the other side of the same trade: the contract with more gamma is the one shedding more value per unit of time.
Two things break the tidiness of that ratio. Implied volatility is not usually equal across the two expirations, and the front expiration frequently prices differently from the one behind it. And a calendar-time clock and a trading-time clock disagree about how much of the overnight gap counts, which is a modelling choice rather than a fact. The ratio is a scale, not a measurement.
The gap itself
A 0DTE position resolves inside a single continuous session. A 1DTE position is held across a window where the S&P 500 futures keep trading while the option's own liquidity is thinner or absent, so the position's value can be repriced by the time it can next be transacted.
That is the structural difference that no Greek captures. Gamma describes what happens per unit of move. It says nothing about whether the move arrives in increments a hedge can follow or in a single jump at the reopen.
Where the two vanish from the data
This is the difference most people never see, and it is the one that matters for anyone reading positioning tools.
Dealer gamma figures, walls and flip levels are built from settled open interest, which is a snapshot taken after each close. A contract has to be open at that moment to appear in it.
- A 1DTE opened today is open at tonight's settlement, so it appears in tomorrow's file, and tomorrow it is the front expiration. Today's 1DTE book is tomorrow's 0DTE book.
- A 0DTE opened and closed inside today's session was never open at any settlement, so no file records it. It contributed volume and it contributed real hedging while it existed, and it left no trace in the open interest that positioning levels are computed from.
Volume versus open interest covers the general version of that distinction. The consequence specific to same-day trading is that the front-expiry levels a site publishes describe the book traders carried into today, not everything they have done since the bell.
How large the same-day slice actually is
One session, measured, so the scale is not a guess. From the delayed Cboe chain captured at 3:14pm Eastern on 2026-09-22, against open interest settled on 2026-09-21:
- The front expiration, expiring that afternoon, had 389 contracts carrying open interest, with gamma at 229 strikes. Between them those contracts held 350,350 contracts of open interest. Summing the gamma the chain publishes for each contract, that expiration carried a net dealer gamma of about 18.3 billion dollars per one percent move in spot.
- The whole chain, front expiration included, was 30,394 listed contracts across 57 expirations, held or not, with gamma and open interest at 682 strikes. Summed the same way, it carried about 126.6 billion dollars per one percent move.
The contract counts are not the same quantity, and one cannot be subtracted from the other. The 389 counts only front-expiry contracts that anybody held; the 30,394 counts every listed contract, one strike, one expiry, one right, whether or not anybody holds a position in it. Open interest counts the positions held in those contracts, so a single contract routinely carries hundreds or thousands of them. That is why a count in the hundreds sits beside a count in the hundreds of thousands on the same expiration.
The dollar gamma figures are the pair that compares, and only because both come from the same computation on the same chain: open interest times the gamma the chain publishes, signed for the dealer side. On that capture the front expiration was about 14.5 percent of the chain's total, so the book that stopped existing at that day's close held about a seventh of it. The front-expiry block on the free SPX gamma page computes the same expiration differently, re-pricing each contract's gamma with Black-Scholes from its own implied volatility, and on the same capture that gave about 16.9 billion. Setting that figure against the 126.6 billion would compare two computations, not two books. Both totals move through the session as spot moves, so the ratio is a scale for the day rather than a constant.
What rolls, and what does not
Every session, the 1DTE book becomes the 0DTE book, and a new 1DTE book is listed behind it. The levels computed from the front expiration therefore reset daily rather than drifting, which is why the front-expiry flip can travel a long way in a session while the all-expiry flip barely moves.
What does not roll is the assumption underneath all of it. Dealer positioning is inferred from open interest plus a convention about who holds which side, and it can be wrong on either expiration. The free SPX gamma page carries a front-expiry block beside the all-expiry levels. As of 2026-09-22 the front-expiry flip and walls in that block are withdrawn, because an audit found the flip was being located by rounding noise in the tail of the ladder and both walls were collapsing onto the strike next to spot. The open interest, the strike count and the net gamma on that expiration are still measured, and the block on the page shows the withdrawal notice in place of the levels.
Educational content, not financial advice. See our risk disclosure.