Why is theta negative? i thought theta is the time value to option expiration? So it must have a positive value right? It’s just decreasing as time passes and eventually ends at 0 when option expires. Am I understand this thing wrong?

Theta is how much an option is dying in value due to the passage of time, therefore it’s negative. Your option might be worth $1 today, but only $0.98 tomorrow (holding everything else equal). Theta is -$0.02.

So you’re correct in thinking that it measures something to do with time value but it’s measuring the decay in the time value, not the time value component itself.

Think of it like your car insurance premium. You bought an insurance premium for 1 year for $1200. The theta on this is -$100/month. In 1 month, if you cancel your insurance you’ll get back only $1100.

Thank you