Long Calendar Spread with Puts Option Strategy

Selling a near dated put and buying a longer dated put at the same strike is the calendar this workbook prices. The front leg decays faster than the back leg, which is the source of the gain when the underlying stays near the strike, and what remains after the short leg expires is simply the longer put.

Fifteen day volatility sits beside the quotes because a calendar is long vega: rising implied volatility helps the longer contract more than it hurts the shorter one. Both legs price from on demand bid, ask and previous close.