A daily market return is only meaningful when the price and comparison date are clear. A Friday close repeated in a Sunday briefing is still Friday’s observation. It is not a new Sunday move.
Start with the two observations
For a price index, calculate the return as (latest completed close / previous completed close − 1) × 100. The comparison must use adjacent trading sessions for that instrument, rather than the first price in an arbitrary download window.
For example, the retrieved Yahoo Finance KOSPI history gives 5,668.0498 for 16 October 2026 and 5,554.2202 for 15 October. Dividing these closes gives a rise of about 2.05%. The result describes that particular session pair. It does not describe a move on the date this guide is published.
Why the sign can change
A return calculated against an older, higher observation can be negative even when the latest one-day return is positive. Both calculations may be arithmetically correct, but they answer different questions. Calling a multi-session change a daily change changes the story readers take away from the same closing level.
Before using a downloaded field called previous close, check the provider’s field definition and the returned dates. Recalculate the percentage from the daily history. Do not assume that a chart’s comparison baseline is the immediately preceding trading session.
Keep publication dates separate from session dates
Store the observation date, previous observation date, exchange timezone and retrieval time alongside each value. On weekends and holidays, retain the last completed session label. During an open market, distinguish an intraday quote from a completed daily close. Mixing the two can create a misleading comparison even if every number was downloaded correctly.
Cross-market comparisons need another check
Seoul and New York close at different times. Matching calendar dates does not make the observations simultaneous, and a US move may occur after Korean trading has ended. A useful briefing identifies the sequence before proposing that one market caused another to move.
Currency direction also matters: USD/KRW is quoted as won per dollar. An increase means more won are required to buy a dollar. State the quotation convention before interpreting the currency move. For bonds, distinguish a change in yield measured in basis points from a percentage change in the yield itself; neither is the bond’s price return.
Use the right instrument
A futures series is not a spot-price series. WTI and gold futures may also roll from one contract to another. CME explains how expiring futures positions move to later contracts. When a change looks unusual, check the contract and roll treatment before treating the series as evidence of a sudden change in physical supply or demand.
A short verification routine
- Identify the symbol and whether it represents an index, FX quote, yield or futures contract.
- Confirm that both observations belong to completed, adjacent sessions.
- Recalculate the percentage or basis-point change from the raw values.
- Record dates, timezones and the source link beside the calculation.
- Separate what the data show from explanations that require additional evidence.
The dated provider observations below illustrate this process; they are historical examples, not current market quotes. Arithmetic checks do not independently confirm the provider’s underlying prices. Missing data, unusual moves and contract changes need further review. This guide explains measurement; it does not recommend a trade.
Market data and observation dates
Completed daily observations from Yahoo Finance. Each row uses its own market timezone and compares the two stated sessions; these are not simultaneous quotes. Futures series can be affected by contract rolls. Provider values have not been independently verified.
| Instrument | Close / yield (%) for TNX | Session | Compared with | Change | Timezone | Source |
|---|---|---|---|---|---|---|
| ^KS11 | 5,668.05 | 2026-10-16 | 2026-10-15 | +2.05% | Asia/Seoul | Yahoo Finance |
| ^TNX | 5.098 | 2026-10-16 | 2026-10-15 | +0.9 bp | America/Chicago | Yahoo Finance |
Worked example in context
See the corrected October 16 market review for how a comparison-baseline correction changes the interpretation of a market session.
