Theta is the reason we use credit spreads. It measures how much the value of an option erodes each day.

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The Short Leg ($700): Theta of -0.132 Decays faster because it is closer to the money.
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The Long Leg ($704): Theta of -0.109 Decays slower.
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The Result: You keep the difference. In our SPY example, the Net Theta is +0.023 (0.132-0.109).
The Bottom Line: You are earning approximately $2.30 per day in “time decay” just for holding the position. Even if the stock market doesn’t move at all, you are getting paid.