Vertical spreads trade strikes. Calendar and diagonal spreads trade time — they pair a near-dated option against a longer-dated one to harvest the difference in decay.
The calendar spread
- Sell a near-dated option and buy a longer-dated one at the same strike.
- The short leg decays faster (theta accelerates near expiry), so time passing tends to help you.
- You’re also long vega — a rise in implied volatility lifts the longer-dated leg more.
Max profit occurs when the stock sits near the strike at the near expiry: the short option expires worthless while your long option retains value.
The diagonal spread
A diagonal is a calendar with different strikes as well as different expirations. This tilts the position directionally — effectively a calendar with a built-in lean. The “poor man’s covered call” (long a deep-ITM LEAPS call, short a near-dated call against it) is a popular bullish diagonal that mimics a covered call for far less capital.
What you’re really betting on
Calendars and diagonals are bets on two things at once:
- Theta — the near leg decaying faster than the far leg.
- The term structure of volatility — how IV differs across expirations. You profit if the front-month IV stays high (or the back-month rises).
The risks
- A big, fast move away from the strike hurts a calendar — both legs go deep ITM/OTM and the time-value edge collapses.
- Vega works both ways: a volatility crush in the back-month leg can sink the trade even if direction is fine.
When to use them
In low-to-moderate IV environments where you expect the stock to hover and front-month options to keep decaying. They’re precision tools — reward patience and a view on time and volatility, not just direction.