I bought a same-day put and a next-day put on the same view — one finished at −17%, the other at −100%

Buying same-day (0DTE) options looks appealing. They are cheap, and when you are right they go up several times over.

I thought so too. Which is why I wrote myself a rule yesterday.

"When I buy a single option, I do not use same-day expiry. Time decay is too steep."

I broke it the next day. Here is what that cost, in numbers.

It turned into a controlled experiment by accident

I expected the index to fall, so I bought two puts. Same morning, same directional read, same reasoning.

One thing differed: the expiry.

	Expiry	Entry → exit	Result
7,700 put	next day (1DTE)	21.10 → 17.60	−17%
7,670 put	same day (0DTE)	4.60 → 0	−100%

The direction was right. The index did fall, and the close landed where my morning thesis said it would.

The same-day put still went to zero.

Why they split like that

An option's price has two parts.

	What it is	At expiry
Intrinsic value	what exercising right now actually leaves you	it stays
Time value	the value of "there is still time, it could get there"	it goes to zero

An out-of-the-money option is entirely time value. Its intrinsic value is zero.

So a same-day out-of-the-money option leaves nothing behind if price does not reach the strike by the close. Whether the direction was right does not enter into it. Only arrival does.

The next-day put had another day. Even with the direction right and price not yet at the strike, there was still time value left to sell into. That is why exiting at −17% was possible at all.

⚠️ "It's cheap, so the downside is small" is the trap

The 7,670 put cost 4.60. The 7,700 put cost 21.10. Much cheaper.

It looked cheap. It was cheap for a reason.

cheap  =  the market did not think price would get there
       =  low probability of arrival
       =  and no time left either

Two things stack. Low probability, and no time to be wrong in.

The session's actual low was 7,688.63 — 18 points short of 7,670. The direction was right, the distance was not, and expiry was that same day, so there was no time to wait.

Entry time made it worse

I bought that put at 06:33 (PT) — right at the open.

There is a common misreading here. "Same-day options are cheap because there is barely any time left" is a late-session statement.

Buying at the open means buying six and a half hours of time value that is about to melt. You pay the most expensive time value there is, then watch it drain all day.

So is same-day expiry always bad? No — the structure is what differs

This is not a case for avoiding 0DTE. The same day, I ran credit spreads on same-day expiry and those finished profitable.

The difference is whether time is on my side or not.

	As time passes
Long a single option	the value shrinks. Time is the enemy
Short the option (credit)	that shrinking is the gain. Time is the ally
A butterfly and similar structures	the debit is a fixed maximum loss, and the wings carry value on the approach

Same-day expiry is not the risk. "Same-day expiry and long a single" is. It is the combination where time turns against the position fastest.

So my rule changed

Yesterday I wrote this.

When I buy a single option, I do not use same-day expiry

Today I rewrote it.

I do not buy same-day single calls or puts. No exceptions.

I removed the room for exceptions for one reason. Yesterday's rule left room, and I walked out through that room in a single day.

What the lesson cost

The day's total loss was −$345. This one position lost −$460.

realized P&L for the day          −$345
the same-day put alone            −$460
────────────────────────────────────────
without that one position         +$115

Breaking the rule cost more than the entire day's loss. Following it would have made the day profitable.

What I do not know yet

• The sample is one position. The numbers above are one day's record, not a statistic. I have never measured the expected value of same-day long singles.
• That said, time value going to zero at expiry is not a statistic — it is the definition of the contract. That part needs no sample.
• What I do not know is whether there are conditions under which a same-day long single is favorable anyway. If there are, I have not found them yet.

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This is a record of option time decay and of my own trading rule. It is not a recommendation to trade any security, strike, or structure. The sample is one day. Your decisions and their outcomes are your own.