Markets around the world keep different clocks, and some never stop. To compare anything across them a system needs one fixed reference. For US equities the most useful one is the regular cash close.
Three clocks
A trading system usually juggles three. Exchange time is the local time of the market, which for US stocks is Eastern Time. UTC is the world reference and has no daylight saving. Local time is whatever the reader’s device shows. A timestamp that does not say which one it uses is ambiguous by several hours.
The common practice is to store every time in UTC and convert only when displaying. Stored that way, two events can always be put in order. Stored in local time, they cannot.
Daylight saving
Eastern Time shifts by an hour twice a year. The US stock market opens at 9:30am Eastern throughout, so its opening time in UTC moves when the clocks change. Other countries change on different dates or not at all, so for a few weeks each spring and autumn the gap between New York and London is an hour off its usual value.
Crypto venues tend to run on UTC and ignore all of this. A daily candle on a crypto venue typically closes at midnight UTC, which is evening in New York. A day on a crypto chart and a day on a stock chart therefore cover different hours.
Why a day’s change differs from the change since the close
A market that never closes has no natural daily boundary, so crypto screens use a rolling 24-hour change. Stocks use the change since the prior close. On a stock perp both exist and they can differ a lot. On a Sunday the rolling figure covers Saturday into Sunday. The change since the close covers everything since Friday afternoon.
For someone thinking about the stock market reopening, the second is the one that matters. It measures the distance from the last price the cash market agreed on.
The cash close as an anchor
The regular session ends at 4:00pm Eastern. On an early-close day it ends at 1:00pm. That moment is the most widely quoted price of the day and the reference for the next day’s change. Anchoring to it gives a fixed point that survives weekends, holidays and clock changes, as long as the system knows the calendar. The implied open on holidays and early closes covers the calendar cases.
NoVo measures each index perp’s price now against the perp’s own price at the last regular cash close, taken from the 15-minute candle that ends at the bell. If the perp has no print at the bell, there is no figure. Since the cash close explains why the perp is measured against itself.
Mistakes worth knowing
A few recur. Using the device’s local midnight as the day boundary, so readers in different places see different daily changes. Hard-coding the UTC offset for New York and being an hour wrong for part of the year. Treating a holiday as a normal session and anchoring to a close that never happened. Mixing a UTC candle from one venue with an Eastern candle from another and calling both today.
What to check
On any screen, find out which clock it uses and what a change is measured from. If neither is stated, assume nothing. A percent change needs a base and a window makes the general case. Trader shows the index perps’ move since the last cash close while the cash market is shut.