What a 0DTE day is
A 0DTE option is one that expires the same session it trades — zero days to expiry. Its whole remaining life is measured in hours, and that short clock changes the physics of the contract. Two of the greeks stop being background numbers and start setting the day's character.
The first is gamma — the rate at which delta changes. With almost no time left, an at-the-money option's delta no longer drifts; it snaps. A small move in the underlying can flip an ATM contract from behaving like almost nothing to behaving like the underlying itself. That is what "gamma dominance" names: near expiry, convexity is large, so directional exposure swings fast for small moves.
The second is theta — the price of time. On a normal option, decay is a slow bleed. On a 0DTE option, all remaining extrinsic value has to reach zero by the close, so it drains steeply and without pause. By the final bell every contract is worth its intrinsic value or nothing — that resolution to a single number is the pin.
A worked example
Take a single generic index session — call the instrument GENCO — trading near 500, and follow the 500-strike options through the day.
At the open, outcomes are widest: hours remain, so both the 500 call and the 500 put still carry extrinsic value, and their deltas sit near the middle. Around midday, price and time pull in opposite directions — every hour that passes subtracts time value, while each move in GENCO re-prices how likely the strike is to finish in the money. Because gamma is large, that likelihood — and the option's delta — can move sharply on a modest push. Into the close, the extrinsic value that is left collapses toward zero, and the contract converges on what it is actually worth at expiry: if GENCO settles above 500 the call has intrinsic value and the put expires worthless; below 500, the reverse. That convergence to one number is the pin.
Nothing here is a rule about when to hold or when to leave. It is the shape of the day: wide and time-rich at the open, gamma-sensitive through the middle, resolving to intrinsic value at the pin.
See it in kestrel
You do not have to take the lifecycle on faith. Run a real, recorded intraday session and read the tape straight off a deterministic replay:
npx kestrel.markets sim fomc-rate-decision-whipsawThat runs a deterministic simulation over a generic, fast-moving index session — managed licensed data, no wall time, no signup, no card — and prints a certified proof URL. Point the CLI back at any proof and it recomputes the whole record on your own machine, byte for byte:
npx kestrel.markets certify https://kestrel.markets/proof/art_d29415f0cf502f4a218a9cbaKeep the tape one command away: drop the kestrel.markets MCP server into your client and the next session you want to read is already wired up — no account in between.