Guide
Zero-day and one-day labels are calendars. This guide separates the clocks — and the confusion that often follows — without turning clocks into trade instructions.
Educational and informational only. Not investment advice.
01
Expires on the same trading day. The label is a countdown, not a strategy.
Expires on the next trading session. Still short-dated; the clock includes an overnight/next-open interval.
For fuller definitions, see What is 0DTE (and 1DTE)?.
02
Because 1DTE still has a session boundary ahead, conversations often mention gap or next-open context. That is a calendar fact people discuss — not a recommendation to hold or flatten anything.
03
Publishers may monitor intraday and next-session option market structure because both calendars are active in modern SPY/SPX listings. Referring to both clocks describes the field of observation.
04
Less time to expiration is a mechanical feature of short-dated options. Educational material often mentions time decay near expiration as a reason for caution and clarity — not timing cues. See Short-dated options risk basics.
05
Knowing which clock is in play does not tell you what to buy or sell. See Market conditions vs trade alerts.
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