How to price a calendar spread option?

How do you price calendar spread options, that is, options on the same underlying and the same strike but different times to maturity?

Clarification: I'm interested in the pricing of a a CSO (calender spread option) as defined by CME Group. Following their definition, the payout of the option at expiry is (price of long nearby futures contract (e.g. September 2008) - price of deferred futures contract (e.g. September 2009) ). (Thanks to user508 for the link and clarification below).

This means the question is still open, and I would be really interested in an answer.

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Notice that in their paper Analytic Approximations for Spread Options, the authors define a spread option as follows:

A spread option is an option whose pay-off depends on the price spread between two correlated underlying assets.

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I think the definition that the CME has is not what is mentioned here. The pricing is actually occurring on the spread value (between two DIFFERENT underlying futures). Therefore, you can have negative strikes and negative underlying values as inputs into you pricing model equation.

I don't think it's as simple as pricing two separate expirations with same strike and underlying.

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First, a quote from Hull:

"A calendar spread could be created by selling a call option with a certain strike price and buying a longer maturity call option with the same strike price"

That means that you can decompose the calendar spread option as a combination of vanilla plain options, which you can price with B-S, trees etc.

For instance, you succeed to represent your calendar spread option in the way the quote above directs, with $C_1$ being a call option with a shorter maturity $T_1$ to be selled and $C_2$ being a call option with a longer maturity $T_2$ to be bought.

Then, Price(calender spread option) = Price($C_2$) - Price($C_1$)

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