How is DTE counted, and why does one day matter more late?
What does days to expiration mean?
Days to expiration, almost always shortened to DTE, is the number of calendar days left until an option expires. It governs how much time value is still in the price and how fast that value comes out. Fewer days mean less premium, faster decay and markedly more gamma risk.
Calendar days over 365
DTE is the number of calendar days until expiration, weekends and holidays included. In the formulas the time left is DTE ÷ 365, the convention used on the expected-move page, in the probability-of-profit calculator and in the decay curve of the IV percentile calculator. Platforms differ in small ways, for example in whether the current day counts, and with only a few days to go a difference of one day is a noticeable share of what is left.
Range and premium grow with the square root
The price range the market prices in grows with the square root of the time left, not in proportion to it. The one-standard-deviation move is the price times IV times the square root of (DTE ÷ 365), and an at-the-money premium follows the same scaling. Four times the days therefore buys about twice the range and twice the premium. Doubling the days adds only about 41 %.
Why one day weighs more the shorter it gets
Because the scaling runs through a square root, removing one day takes a larger share the fewer days are left. From 7 to 6 days the at-the-money premium falls by about 7 %, since the square root of 6/7 is 0.926. From 28 to 27 days it falls by under 2 %. A position with a week to go therefore changes value faster from the calendar alone than the same position a month out, before the stock has moved at all.
The common misreading
Mixing day counts is the quiet error. Counting 20 trading days where a formula expects 28 calendar days, both four weeks, shrinks the time input and with it every result: at an IV of 30 % on a 100 € stock the one-standard-deviation range comes out at about 7.02 € instead of 8.31 €. Historical volatility is annualised with trading days and 252; DTE in option formulas uses calendar days and 365. Keep each figure in its own convention, and check which one a platform uses.
Worked example
Hypothetical numbers for illustration, not a market quote and not a recommendation.
An example stock trades at 100 €, and its at-the-money options trade at a hypothetical implied volatility of 30 %, rates ignored. With 7 days to expiration, the one-standard-deviation range is 100 × 0.30 × √(7/365) ≈ 4.15 €, the call costs 1.66 € and its gamma is 0.096. With 28 days the same figures are 8.31 €, 3.31 € and 0.048.
Four times the days give about twice the range and premium and half the gamma. One day less cuts the 7-day call from 1.66 € to 1.53 €, about 7 %, and the 28-day call from 3.31 € to 3.25 €, about 1.8 %. Fees are ignored.
Strategies where the term matters
- Cash-Secured Put — After a 30% drop, would the strike still be a price you actually want to pay?
- Short Iron Condor — Is the range you expect genuinely narrower than the move the market has priced in?
- Long Call Calendar — Is the near-term expiration expensive relative to the far one (a term structure in backwardation)?
Calculators for this term
Track it in your journal
Related terms
Every term in one place: the options glossary.
This material is general information about option strategies, written for a general audience. It is not investment advice, not a recommendation, and not financial analysis. Options can lose their entire value, and uncovered positions can lose more than the amount committed. Anything said about tax is general in nature and is no substitute for professional tax advice. Last reviewed: 2026-08-06.