Calendars and Diagonals
Calendar spreads are bets on time decay differentials and term-structure shape. They are the most popular structure that almost no one understands well.
Calendar spread basics
A calendar spread (also called a horizontal spread or time spread) is two options of the same type at the same strike, but with different expiries. Sell the front-month, buy the back-month. The position is a long calendar spread. The reverse (sell back, buy front) is a short calendar, which is rarely used by retail.
The structure profits from the front-month decaying faster than the back-month. Because theta accelerates as expiry approaches (covered in L.07), the front month loses value more rapidly than the back month, generating a small profit each day if the underlying stays near the strike.
The shape of the P&L curve
At front-month expiry, the calendar's value depends on where the underlying is. If it sits at the strike, the front-month is worth zero (max time decay collected) and the back-month still has its remaining time value. The position earns its peak P&L.
If the underlying moves significantly away from the strike, both options lose extrinsic value and the front-month either expires worthless (if OTM) or becomes pure intrinsic (if ITM). The back-month's time value also collapses for OTM strikes. The calendar spread bleeds out either way.
Vega exposure
The calendar is long vega. The back-month has more vega than the front-month (longer time means more vega). When IV rises, the calendar generally profits. When IV falls, it loses.
This is why calendars are often opened when IV is at relatively low levels (cheap to buy vega, room for it to expand) and closed when IV spikes. The vega exposure can dominate the theta-collection P&L on days when IV moves.
Diagonal spreads
A diagonal spread is a calendar with different strikes on the two legs. Sell front-month at one strike, buy back-month at a different strike. The structure is more flexible (and more complex) than a pure calendar.
Common diagonals:
- Bullish diagonal call: long back-month ATM call, short front-month OTM call. Profits if underlying drifts up moderately.
- Poor man's covered call: long deep-ITM LEAPS call as a stock substitute, short front-month OTM call against it. Capital-efficient version of a covered call (covered in L.19).
- Bearish diagonal put: long back-month ATM put, short front-month OTM put. Profits on moderate downside drift.
Strike and expiry selection
The standard calendar template uses ATM strikes for both legs. The ATM strike has the most theta differential between the two expiries, which is what the trade is harvesting.
For the expiries: front month is typically 20-35 days. Back month is typically 60-90 days. Wider spreads (front 7 days vs back 60 days) extract more theta per day but expose the position to gamma. Tighter spreads are more sensitive to vega and less to theta.
When calendars do well
Three regime characteristics favor calendars:
- Low front-end IV. Cheap front-month options to sell mean less premium up front but also less risk if the underlying moves.
- Steep contango. Back-month IV well above front-month. The structure benefits from the IV differential.
- Range-bound underlying. Calendars need the spot to stay near the strike. Trending markets push the spot away and kill the structure.
Many calendar traders run them on stocks with predictable mean-reverting behavior, or on indices in low-vol regimes where range-bound action is the norm.
Risk management
Calendars have soft losses and hard losses. The soft loss happens when the underlying drifts moderately away from the strike. The position bleeds slowly as the front-month decays toward the underlying's distance from strike.
The hard loss happens on a vol crush. If IV collapses across the surface (often after an event passes), the long back-month vega loses fast. Calendars opened ahead of earnings or FOMC can lose more from the post-event vol crush than they make from the spot move staying contained.
Standard risk-management approach: close at 25-50% of debit paid as profit target. Close at 50-100% of debit as max loss. Close before any major event the position was not designed to harvest.
What you carry forward
- Calendar spreads sell front-month and buy back-month at the same strike.
- They profit from theta differential and benefit from rising IV (long vega).
- Maximum profit is at the strike at front-month expiry. Profit drops in both directions.
- Diagonals add a strike differential, which makes the structure more directional.
- Calendars do best in low-vol, range-bound regimes with steep contango.