Day Order

A day order dies at the 4:00 pm ET close if it has not filled, in contrast to a GTC order that keeps working across sessions. It is the default time-in-force at nearly every broker: submit a limit order without touching the duration field and this is what you sent.

The expiry is a feature for active traders, because a thesis built on today's tape should not survive into tomorrow's. Note the boundary: a standard day order does not work in premarket or after-hours; participating in extended sessions requires explicitly flagging the order for extended hours, and those fills carry wider spreads.