Calendar spread (options)

Definition

A calendar (time) spread sells a near-dated option and buys a longer-dated option at the same strike and type. It is a term-structure trade: the position profits when the front option decays faster than the back (their theta differential) and when the term structure moves in its favor — the underlying staying near the strike while near-term priced volatility falls relative to longer-term.

How it works / structure

• Legs: −1 option (call or put) at strike K, near expiry T1; +1 same-type option at K, later expiry T2 (debit structure).
• Cost: net debit D (back leg costs more); maximum loss = D (both legs same strike/type); maximum gain occurs with the underlying at K at T1 expiry — not computable in advance because it depends on the back leg’s IV then.
• Parameters (engine-executable): strike placement (ATM for a pure time/volatility position; offset for directional tilt — that variant is strategy-diagonal-spread when strikes differ), expiry pair (T1/T2 gap, e.g. 30/60 DTE), term-state gate (vol_term_state — entries when the front is rich vs back), event flag (earnings inside T1 vs between T1 and T2 changes the thesis entirely — earnings_iv_crush_pts), exits (mgmt-profit-target, mgmt-time-based-exit at or before T1).
• Greeks profile: long back-month vega, short front-month gamma/theta — net long vega, net positive theta near the strike; the position is long the term structure’s slope.

When it applies

Term-structure theses (front-month IV rich vs back — inverted or event-humped curves, opt-term-structure), pre-positioning for post-event IV normalization (short the event expiry, long the clean one), and pin-at-strike expectations with defined risk (alternative to strategy-iron-butterfly).

Risk profile & failure modes

• Big moves defeat it: a large move away from the strike takes both legs toward intrinsic/zero and the debit is lost — the structure is short realized movement even though it is long vega.
• Back-leg IV is the hidden variable: profits assume the back month holds its IV; a whole-surface crush (post-event, regime calm-down) hits the long leg too and can turn a “correct” pin into a loss.
• Event mislocation: an earnings date landing in the wrong expiry inverts the thesis (event-earnings); date confirmation is a hard precondition.
• Assignment on the short front leg when it goes in the money (ms-expiration-exercise-assignment) leaves a naked back-month option and stock.

Evidence & limits

Mechanics are contract arithmetic (OCC/Cboe). Term-structure slope harvesting has documented evidence in the listed-volatility space (Simon-Campasano 2014, cited in opt-term-structure) with documented drawdown episodes; equity-option calendar programs specifically have no established public excess-return record — replay per underlying and term-state is the platform’s evidence.

Falsifiable-thesis examples

Illustrations only, not signals:

• “X will be within ±3% of strike K at the front expiry” — falsified by the T1 close.
• “X’s front-month IV will fall relative to back-month IV by at least 4 points after the report (term normalization)” — falsified by the IV pair series.

Cross-references

• Strike-offset variant: strategy-diagonal-spread; pin alternative: strategy-iron-butterfly
• The axis it trades: opt-term-structure; exposures: greek-vega, greek-theta
• Event mechanics: event-earnings
• Management: mgmt-profit-target, mgmt-time-based-exit