Calendar and diagonal spreads (India)

🎯 In 30 seconds: A calendar spread sells an option that expires soon and buys the same strike further out. A diagonal does the same with different strikes. The near option loses value faster than the far one, so if the market stays near the strike, the spread gains. These trades depend on implied volatility as much as on price. In India, the margin benefit for offsetting positions in different expiries is removed on the day the near contract expires, for index derivatives since 1 February 2025 and for single-stock derivatives since 2026. Plan for that day before you enter.

Why time can be traded

An option's price has two parts: intrinsic value and time value (see What an option is worth). Time value shrinks to zero by expiry, but not evenly. It shrinks slowly when expiry is far away and fast in the last days.

Think of a sweet shop that sells mithai made today. A box with five days of shelf life left barely loses value overnight. A box that must be sold by tonight loses most of its value by closing time. A calendar is like buying the fresh box and selling the one that expires tonight.

Here is the effect with a pricing model: a 25,000 Nifty call with the Nifty at 25,000, 13% implied volatility, 6.5% interest, and the index held still.

Black-Scholes, Nifty held at 25,000, 13% implied volatility, 6.5% interest, premium in index points. The call loses about 59 points in the week from 35 to 28 days, but about 195 points in its last week. Calendars are built on that difference.

The call calendar, step by step

Nifty weekly options expire every Tuesday on NSE (since September 2025), and the last Tuesday of the month is also the monthly expiry. SEBI's May 2025 circular limits each exchange's equity derivative expiries to Tuesday or Thursday. So there are always several expiries to pair. Hypothetical example, prices from the same model, one lot of 65 units (the NSE lot from January 2026 contracts; check the current lot):

Sell 1 lot of the 25,000 call, 7 days to expiry	+195
Buy 1 lot of the 25,000 call, 35 days to expiry	-483
Net debit	288 points = ₹18,720

On the day the short call expires there are three broad outcomes:

Nifty still at 25,000. The short call expires worthless. Your long call has 28 days left and is worth about 424 points if implied volatility has not changed. That is 424 - 288 = about +137 points, or roughly ₹8,900. This is the best case.
Nifty far below 25,000, say 24,400. The short call expires worthless, but the long call has also lost value. Estimated loss about 122 points, roughly ₹7,900.
Nifty far above, say 25,600. The short call is 600 points in the money and is cash settled against you. The long call is worth a bit more than 600, but not enough. Estimated loss about 53 points, roughly ₹3,400.

Model estimate for one lot of 65, before charges. Unlike a single-expiry spread, this is not an exact payoff: the long call still has 28 days left and its price depends on implied volatility on that day. The loss can never exceed the 288-point debit (about ₹18,700).

For a spread with one expiry, the payoff at expiry is exact. A calendar has two. When the first expires, the long option is still alive, and its price depends on implied volatility at that moment. Nobody knows that number in advance. The chart therefore shows two assumptions: the blue line keeps volatility at 13%, the dashed line drops it to 11%. With lower volatility the peak falls from about ₹8,900 to about ₹5,300 and the profit zone narrows. Treat the breakevens as estimates.

Two things are firm. The maximum loss is about the debit, ₹18,720, because the long call can never be worth less than zero and covers the short call above 25,000. And the profit is highest near the strike on the first expiry day.

Margin, and what changes on expiry day

NSE Clearing uses SPAN, which looks at your whole portfolio. Normally it treats a short near-month option and a long far-month option on the same index as partly offsetting, so a calendar needs much less margin than the short option alone. That offset is called calendar spread treatment.

SEBI's circular of 1 October 2024 removed this treatment on expiry day for index derivatives, from 1 February 2025. On the day the near contract expires, the worst-scenario loss is calculated separately for the contracts expiring that day and for everything else. SEBI's reasoning was that expiring and later contracts can move very differently on expiry day ("basis risk"), and expiry-day volumes are very large.

In practice, on the Tuesday your short weekly call expires:

The short call is margined as if it stood alone, so SPAN on the position jumps.
The extra 2% extreme loss margin on short index options expiring that day, in force since 20 November 2024, applies on top: 2% x 25,000 x 65 = about ₹32,500 on our example.
If your account cannot cover the higher margin, you face a shortfall or a forced exit. Many traders close or roll the short leg the day before, or bring extra money on expiry morning.

Calendars that do not involve the expiring contract keep their treatment. SEBI's example: with monthly expiries on the 29th, 30th and 31st, a spread between the 30th and 31st contracts still gets calendar treatment on the 29th.

For single-stock derivatives, SEBI's circular of 5 February 2026 removed the same benefit on expiry day for contracts expiring that day, effective three months from the circular date. Stock calendars are now handled the same way as index calendars on expiry day.

Margin math is not the same as risk math

The removal of the benefit does not change what the spread can lose. It changes how much money must sit in your account on one specific day. Check your broker's margin calculator with the expiry-day setting, or ask your broker what it will block on that morning, before you enter the trade.

The role of implied volatility

A calendar is long vega: you own the option with more time, and longer options are more sensitive to implied volatility. If volatility rises, the long option gains more than the short one loses, and the spread gains. If it falls, the spread loses, even with the Nifty sitting at the strike.

Implied volatility is not the same for every expiry. Before an event such as an RBI policy announcement or the Union Budget, the expiry just after the event usually carries higher implied volatility than later ones. Some traders sell that expensive near option and buy a later, cheaper one. When the event passes, the near option's volatility drops sharply, which helps. But if the event produces a big move, the price change can swamp the volatility effect. There is no free trade here either.

Practical tip

Before entering a calendar, list every scheduled event between today and the far expiry: RBI policy, Budget, major US data, and for stocks, results and dividend record dates. Each one changes how the spread behaves. Indian options are European style, so there is no early assignment, but a stock calendar whose short leg ends in the money is physically settled at expiry.

Diagonal spreads

A diagonal is a calendar with different strikes: buy a longer-dated option at one strike and sell a shorter-dated option at another. Changing the strikes tilts the spread toward a direction. A long lower-strike call with a short higher-strike call is mildly bullish. A long higher-strike put with a short lower-strike put is mildly bearish.

The best-known diagonal is the poor man's covered call. It copies a covered call, but replaces the underlying with a deep in-the-money call that has a long time to run. On the Nifty you cannot buy the index itself in lots, so the comparison is with a Nifty future plus a short call.

The poor man's covered call on the Nifty, worked

Hypothetical example, Nifty at 25,000, 13% implied volatility. NSE lists quarterly and half-yearly Nifty options as well as weekly and monthly ones (they appear in NSE's October 2025 lot-size circular).

Buy 1 lot of the 23,500 call, about 3 months to expiry (delta about 0.89)	-1,965
Sell 1 lot of the 25,500 call, 7 days to expiry	+36
Net debit	1,929 points = ₹1,25,385

Estimated results on the day the weekly call expires, assuming implied volatility stays at 13%:

Nifty at 25,500: the short call expires, the long call has gained. About +459 points, or ₹29,800.
Nifty at 25,000: roughly breakeven. The 36 points collected offset the long call's small time decay over the week.
Nifty at 24,000: about -823 points, or -₹53,500.
Nifty at 23,000: about -1,427 points, or -₹92,800.

Then you sell another weekly call against the same long call, and repeat.

Leverage cuts both ways. A 4% fall costs this position about 43% of the ₹1.25 lakh you paid. See Leverage everywhere.
Every Tuesday is an expiry day for the short leg, with the extra 2% ELM and no calendar treatment for the expiring call. Rolling weekly means facing that rule every week.
Upfront premium. The full ₹1,25,385 debit is collected before the trade, under SEBI's upfront premium rule in force since 1 February 2025.
Liquidity. Quarterly Nifty strikes far from the money can have wide bid-ask gaps. Check the spread before you trade, not after.

Common mistake

Building a diagonal where the strike width is smaller than the debit. Here the width is 25,500 - 23,500 = 2,000 points and the debit is 1,929, so if the Nifty rallies hard and both calls go deep in the money, the spread is worth at least 2,000 and you keep a small profit. With the 25,300 call instead, the width would be 1,800 against a debit of about 1,888, and a big rally would lock in a loss. Check width against debit before you enter.

Managing calendars and diagonals

Roll the short leg. When the near option is nearly worthless, buy it back and sell the next one against the same long option. Do this the day before expiry if you want to avoid the expiry-day margin jump.
Close before the short leg expires if the Nifty is near the strike. Expiry-day moves in the last hour can swing the short leg sharply.
Watch the long leg's time. Once it has only a few weeks left, it decays fast too, and the spread loses its reason to exist.
Count costs. Each roll is two trades. STT is 0.15% of premium on every option sold from 1 April 2026, plus brokerage and charges. Weekly rolling adds them up quickly.

Try it

On the NSE option chain for the Nifty, choose the at-the-money strike. Write down the mid price of the call for the nearest weekly expiry and for the monthly expiry about five weeks out. Then:

Work out the calendar's debit in points and in rupees for one lot.
Enter the position in your broker's margin calculator and note the margin. If the calculator lets you, check what it shows for expiry day.
Note India VIX today.
On the day the weekly expires, record the Nifty level, India VIX and the monthly call's price, and work out what the calendar would have made or lost. Was the result driven more by the Nifty's move or by the change in volatility?

Check yourself

What is a calendar spread? Answer: You sell an option that expires soon and buy the same strike with a later expiry. The near option loses time value faster.
You sell the 7-day 25,000 Nifty call for 195 and buy the 35-day call for 483. What is the debit per lot of 65, and roughly what is the maximum loss? Answer: 288 points, ₹18,720. The maximum loss is about the debit.
What happens to a Nifty calendar's margin on the day the short leg expires? Answer: Calendar spread treatment is removed for the expiring contract (since 1 February 2025), so the short leg is margined on its own, and the extra 2% ELM on expiring short index options applies.
Why is a calendar long vega? Answer: The longer-dated option you own is more sensitive to implied volatility than the near one you sold, so rising volatility helps the spread and falling volatility hurts it.
What should you check about strike width and debit in a diagonal? Answer: The width should be larger than the debit, or a big move in your favour can still lock in a loss.

🗓️ Calendar and diagonal spreads: trading time itself