How does a diagonal spread exploit time decay differences between near-term and far-term options?

A diagonal spread combines vertical (different strikes) and calendar (different expirations) spread elements to exploit the faster time decay of near-term options relative to longer-dated options. The most common version sells a near-term OTM option and buys a longer-term option at a different strike.

Construction (Bullish Diagonal Call Spread):

• Buy 1 longer-dated call at strike K1 (lower or ATM)
• Sell 1 shorter-dated call at strike K2 (higher, OTM)

The short near-term option decays rapidly (high theta), while the long far-term option retains value (lower theta). The trader profits from the differential decay rate.

Theta Decay Curve:

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Worked Example:

Crestline Aviation trades at $64. Trader Yuki constructs a bullish diagonal:\n\n| Leg | Strike | Expiration | Premium |\n|---|---|---|---|\n| Buy call |$60 | 90 days (July) | -$7.20 |\n| Sell call |$70 | 30 days (May) | +$1.80 |\n\nNet debit:$7.20 - $1.80 = **$5.40**

Scenario Analysis at May Expiration (30 days later):

Scenario A: Stock at $64 (unchanged)*\n- Short$70 May call expires worthless: Yuki keeps $1.80\n- Long$60 July call (60 DTE remaining): worth approximately $6.80 (lost some time value)\n- Position value:$6.80 - $5.40 = **+$1.40 profit*

• Yuki can sell another near-term call to continue the strategy

*Scenario B: Stock at $72 (above short strike)*\n- Short$70 May call: -$2.00 intrinsic\n- Long$60 July call: approximately $14.50 (deep ITM with time value)\n- Position value:$14.50 - $2.00 -$5.40 = *+$7.10 profit**\n\n*Scenario C: Stock at$55 (below both strikes)

• Short call expires worthless: +$1.80\n- Long$60 July call: approximately $2.30 (mostly time value)\n- Position value:$2.30 + $1.80 -$7.20 = **-$3.10 loss**\n\n**Rolling the Short Leg:**\n\nThe key advantage of diagonals is the ability to **roll** the short option forward. After the May expiration, Yuki can sell another 30-day call (June$70) for additional premium, further reducing the cost basis of the long call. Repeating this across multiple cycles can potentially recover the entire initial investment.

Risks:

• Sharp upside moves can cause the short call to go deep ITM, creating assignment risk
• A collapse in implied volatility hurts the long-dated option more (higher vega)
• The expiration mismatch makes delta management more complex

Learn diagonal and calendar spread mechanics in our CFA Derivatives course.