Calendar-spread assignment as a volatility regime shift

The short leg can still be assigned

The short leg of a long calendar, or of any spread that contains short option contracts, can be assigned before expiration even when the position was opened as a spread. A long calendar is a debit spread that is long a later-dated option and short a nearer-dated option of the same type and strike. Assignment converts that short option into the corresponding stock position when the counterparty exercises.

Early assignment on a long call calendar is uncommon in ordinary conditions because holding the remaining long call is cheaper than carrying the equivalent long stock. Exercise becomes attractive when an ex-dividend date or a hard-to-borrow stock changes that comparison. A hard-to-borrow name raises the cost or risk of holding a short-stock leg after assignment. An ex-dividend date is a common catalyst for early exercise of calls because a stock buyer after that date is not entitled to the next dividend.

When a deep long put is exercised

A long put is theoretically exercised when the carry-cost of the equivalent long-stock-plus-long-put combination exceeds the listed call of the same strike. That typically happens once the put is deep in the money and the matching call has little or no leftover premium. Carry-cost here is the financing or holding cost of a stock-plus-option combination versus the listed option of the same strike.

When exiting a long put, selling in the listed market is preferable to exercise if the sale captures value above parity that exercise would forfeit. Parity is an option price that equals only its intrinsic value, with little or no remaining extrinsic value. Extrinsic value is the portion of an option price above intrinsic value, driven by time and implied-volatility.

What the book becomes after assignment

If the short call of a long call calendar is assigned, the book becomes a synthetic long put, typically short stock plus a long call of the same strike. Maximum loss remains the original calendar debit, while the payoff can expand if the underlying falls.

Any assignment profit on a call calendar largely tracks leftover premium in the same-strike listed put. The put-calendar analogue compares the same-strike back-month call with the synthetic long call created at the calendar's cost. A synthetic long call is typically long stock plus a long put of the same strike. Implied-volatility, the volatility priced into listed options, is the yardstick for whether leftover premium survives after assignment.

A mid-June mark-to-market illustration

In a mid-June illustration with the underlying at 59.32, a four-days-to-expiry July/June 55 call calendar marked at 1.83 would, after assignment, become a synthetic July 55 put costing 1.83 versus a listed July 55 put at 1.97. That is a 0.14 mark-to-market gap if price and implied-volatility were unchanged.

An extreme assignment on a July/June 60 call calendar marked at 2.46 against a listed July 60 put at 3.90 would show a 1.44 paper gain. That outcome is economically similar to collecting the June 60 put's 1.48 leftover premium without remaining short that put.

Assignment on either calendar type can produce a quick mark-to-market gain while leaving the original debit as the capital at risk. It replaces calendar risk with higher margin and with the directional, time-decay, and implied-volatility exposures of a long put or long call.