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.\n\nConstruction (Bullish Diagonal Call Spread):\n\n- Buy 1 longer-dated call at strike K1 (lower or ATM)\n- Sell 1 shorter-dated call at strike K2 (higher, OTM)\n\nThe 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.\n\nTheta Decay Curve:\n\n`mermaid\ngraph TD\n A[\"Time Decay Relationship\"] --> B[\"Near-Term Option
30 DTE
Theta = -$0.08/day\"]\n A --> C[\"Far-Term Option
90 DTE
Theta = -$0.03/day\"]\n B --> D[\"Decays 2.7x faster\"]\n C --> D\n D --> E[\"Net theta = +$0.05/day
Diagonal trader earns
$0.05 per day from decay\"]\n`\n\nWorked Example:\n\nCrestline 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\n\nScenario Analysis at May Expiration (30 days later):\n\nScenario 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\n- Yuki can sell another near-term call to continue the strategy\n\nScenario 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\nScenario C: Stock at $55 (below both strikes)\n- 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\nRolling 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.\n\nRisks:\n- Sharp upside moves can cause the short call to go deep ITM, creating assignment risk\n- A collapse in implied volatility hurts the long-dated option more (higher vega)\n- The expiration mismatch makes delta management more complex\n\nLearn diagonal and calendar spread mechanics in our CFA Derivatives course.