Calendar Spread

Core Thesis

A Calendar Spread trades time dispersion and term-structure convexity rather than pure direction. You sell faster-decaying near-term optionality and buy slower-decaying longer-dated optionality at the same strike.

Structure

• Short front-month option at strike $K$.
• Long back-month option at strike $K$.
• Net debit $D$ at entry.

Payoff Reality

Unlike verticals, static expiration formulas are less informative because one leg expires earlier. Risk and reward are best evaluated at front-leg expiration $t_1$:

$$\Pi_{t_1} = V_{\text{back}}(S_{t_1}, \sigma_{t_1}, T_2-t_1) - \max(\pm(S_{t_1}-K),0) - D$$

• Max loss is generally bounded by entry debit $D$.
• Peak outcome usually occurs when spot is near strike at front expiry and back-month IV is stable/richer.

Greek and Regime Profile

• Typically near-delta-neutral at entry.
• Net theta can be positive near strike, negative when spot drifts far.
• Net vega is positive because back-month option carries larger vega.

Design Rules

• Choose strike where you expect spot to gravitate by front expiry.
• Use maturities with meaningful decay differential (for example 25-45 DTE short vs 60-120 DTE long).
• Evaluate front vs back IV spread, not just absolute IV.

Management Framework

• Manage around first expiration: close, roll short leg, or convert to directional structure.
• Avoid passive assignment risk on expiring short options.
• Reassess after catalysts; term structure can reprice sharply.

Failure Modes

• Treating calendars as static range trades when term structure is unstable.
• Selling too short-dated front legs into binary events.
• Ignoring liquidity/slippage in back-month leg adjustments.

Practical Checklist

• Is expected spot location near strike at front expiry?
• Is term-structure edge present, not just headline IV level?
• Is there a defined action plan for short-leg expiration week?