Trading hours across the three markets, and the trap in comparing them
The Korean, US and Hong Kong sessions barely overlap. This page lists the hours, explains how the site decides a market is shut, and why cross-market spreads are labelled as being based on each market's most recent valid quote.
The three sessions
The Korea Exchange runs 09:00 to 15:30 KST with no midday break. The Hong Kong exchange runs 09:30 to 16:00 HKT with a lunch break from 12:00 to 13:00. Nasdaq runs 09:30 to 16:00 Eastern, which shifts by an hour against UTC with daylight saving. Put on one timeline, the US market is still about six hours from opening when Korea closes, and Korea is already into the next morning when the US closes. At any instant at most two of the three are trading, and usually only one.
How the site decides a market is shut
Going by the timetable alone gets holidays wrong: the clock matches, but nothing traded. Rather than maintain three holiday calendars — which need updating every year and still miss unscheduled closures — the site infers it from quote freshness. An authoritative status from the data source wins when one is available. Otherwise, if a quote inside nominal hours has not advanced for more than fifteen minutes, or its timestamp still sits on the exchange's previous calendar day, the market is treated as shut and the last valid quote time is shown as it is. Holidays, unscheduled closures and feed failures are all covered by the same rule.
Why the spread carries a reference time
Computing a premium from Korea's close yesterday and the US price right now produces a number containing everything that happened in between. That is not wrong, but the reader has to know. So the premium block always carries the line that cross-market comparison uses each market's most recent valid quote, and lists the actual quote time for each market beside it. Once you can see that two quote times are fifteen hours apart, it is obvious the spread is not a synchronised snapshot.
What that means in practice
A cross-market spread is most reliable while the sessions overlap, and closer to one market's expectation of the other's next open when they do not. The same premium value therefore means different things at different times of day. When watching it move, first check whether the two quote times are in step, then decide whether the move reflects a genuine difference of opinion or merely one side not having updated yet.
This page documents how this site computes the figure. It is not investment advice.