Time-in-force is how long a working (unfilled) order remains active. The two you'll meet most are Day and GTC, and they answer one question: if my order doesn't fill, when does it die?

Day vs GTC

A Day order is good only for the current session — if it hasn't filled by the close, it's cancelled automatically. A GTC (“good-til-cancelled”) order keeps working across future sessions until it either fills or you cancel it (brokers usually cap it at 30–90 days). GTC is handy for resting orders at a level you might not reach today.

Why it's nearly moot on 0DTE

On a same-day option, the contract itself expires at the close, so a working order on it has nothing to be good for tomorrow — the instrument is gone. Practically, Day and GTC behave the same on a 0DTE: the order and the option both end their life at the bell. The distinction matters on longer-dated options, where a GTC entry can sit at a level for days.

On 0DTE, the option outliving your order isn't the issue — the option doesn't outlive the day at all.

The one thing to watch

The trap is a stale GTC order on a longer-dated position that you forgot about — it can fill days later at a price that no longer makes sense. If you use GTC, keep a list of what's resting. For same-day scalping, Day orders are the natural default, and your bracket/OCO exits handle the getting-out. Pair the right time-in-force with the right fill condition and your orders behave exactly as you intend.