Understanding Time Decay in Options

Time Is Money in Options

In options trading, time has a direct impact on the value of an option.

Consider a simple example.

Suppose you are preparing for an important competitive examination. If you have 30 days to prepare, you have plenty of time to revise and improve your chances of success.

If you have only one day left, the opportunity to improve becomes much smaller.

The same idea applies to options.

An option with more time before expiry has a greater opportunity for the underlying asset to move enough for the option to become profitable.

As expiry approaches, that opportunity reduces.

Time and the Probability of Expiring ITM

Suppose the underlying is trading at 8,500.

You buy an 8,700 Call Option.

For this Call to expire ITM, the underlying needs to move 200 points above the current level.

Now consider different amounts of time remaining.

30 Days to Expiry

There is considerable time for the underlying to move 200 points.

The likelihood of the option expiring ITM is relatively high.

15 Days to Expiry

There is less time, so the likelihood is lower than with 30 days.

5 Days to Expiry

A 200-point move becomes more difficult within such a short period.

The likelihood reduces further.

1 Day to Expiry

The underlying has very little time to make the required move.

The likelihood of the option expiring ITM becomes very low.

The key idea is simple:

More time generally means a greater opportunity for an option to become ITM.

What Happens as Expiry Approaches?

As an option gets closer to expiry, the amount of time value remaining in the premium decreases.

This process is known as Time Decay.

All else being equal, an option loses some of its value as each day passes.

This is especially important for options that are OTM because their value depends heavily on the possibility of the underlying making a favourable move before expiry.

Understanding Time Value

An option premium can be broadly understood as:

Premium = Intrinsic Value + Time Value

Intrinsic Value

The value the option already has based on the current underlying price.

Time Value

The additional value investors assign to the possibility that the option may become more valuable before expiry.

As expiry approaches, this time value gradually reduces.

At expiry, there is no time value left.

The option is worth only its intrinsic value, if any.

What Is Theta?

Theta measures the impact of the passage of time on an option's premium.

In simple terms:

Theta tells us how much option value is lost as time passes, assuming other factors remain unchanged.

Theta is therefore commonly associated with time decay.

For an option buyer, time decay generally works against the position.

For an option seller, time decay generally works in their favour.

Theta for Option Buyers

Suppose you buy an option today.

Even if the underlying price does not move, the option may become less valuable tomorrow because there is now one less day available for the expected price movement to happen.

This means the buyer is paying for time, and that time is continuously running out.

Therefore:

Long Options → Time Decay Works Against You

Theta for Option Sellers

The situation is different for an option seller.

When an option seller receives a premium, part of that premium represents time value.

As time passes, this time value gradually decreases.

If other factors remain favourable, the seller can benefit from this decline.

Therefore:

Short Options → Time Decay Generally Works in Your Favour

This is one of the important reasons option sellers pay close attention to Theta.

Time Decay Is Not Uniform

Time decay does not happen at the same rate throughout the life of an option.

When there is plenty of time remaining:

• The option has more time value.
• Daily time decay is relatively slower.

As expiry approaches:

• Time value becomes smaller.
• The rate of time decay becomes faster.

So an option can lose value relatively slowly at the beginning and much faster as expiry gets closer.

A Practical Example

Consider two identical options:

Option A

30 days remain until expiry.

Option B

5 days remain until expiry.

If the underlying price and other factors remain unchanged, Option B is generally more sensitive to the passage of another day because very little time remains.

This is why traders need to consider time to expiry, not just the option premium.

Why Option Sellers Like Theta

Option sellers are effectively being compensated for taking the risk associated with the passage of time.

At the beginning of an option series, premiums may contain substantial time value.

However, the daily reduction in premium due to Theta is relatively slow.

As expiry approaches, the remaining time value becomes smaller, but the rate at which it disappears becomes much faster.

This creates an important trade-off for option sellers:

More Time → More Time Value but Slower Decay

Less Time → Less Time Value but Faster Decay

Practical Insight

When evaluating an option, always ask:

• How many days are left until expiry?
• How much of the premium is intrinsic value?
• How much is time value?
• How quickly could time decay affect the premium?

An option that looks attractive today may become less attractive simply because the clock is moving against it.

Common Beginner Mistake

A common mistake is assuming that an option's premium will remain unchanged if the underlying price does not move.

It will not necessarily remain unchanged.

Even when the underlying stays at the same price, the option premium can decline because time is passing.

Key Insight

An option is a wasting asset because its remaining time keeps reducing.

For buyers, time decay is generally a disadvantage. For sellers, it can become an important source of potential profit.

Key Takeaways

• Time is an important component of an option's value.
• More time before expiry generally gives an option a greater opportunity to become ITM.
• Premium = Intrinsic Value + Time Value.
• Time value gradually decreases as expiry approaches.
• Theta measures the impact of time decay on an option's premium.
• Time decay generally works against option buyers.
• Time decay generally works in favour of option sellers.
• Time decay is relatively slow when plenty of time remains.
• Time decay accelerates as expiry approaches.
• At expiry, an option has no time value remaining.