US stock pre-market and after-hours trading are extended trading sessions outside of regular hours. The corresponding China Standard Time (CST) varies depending on Daylight Saving Time (DST), and there is no unified official price. Below is a detailed introduction.

1. Pre-market and after-hours trading sessions

In addition to regular trading hours, US stock markets offer extended hours, including the pre-market and after-hours sessions. The times for these sessions shift by one hour in the GMT+8 time zone to accommodate US DST. Eastern Daylight Time (EDT) starts on the second Sunday of March, and Eastern Standard Time (EST) resumes on the first Sunday in November.

2. Price mechanism in pre-market and after-hours sessions

During pre-market and after-hours trading, US exchanges do not publish a unified official price. Brokers update the pre-market/after-hours high, low, and last sale prices based on the latest fill prices and their specific rules.

Although the exchanges do not provide official prices for the extended hours, there are official guidelines. We provide the pre-market and after-hours prices of US stocks based on the guidelines in official documents from Nasdaq and NYSE.

A crucial rule is that a fractional share fill does not update the high, low, or last sale prices.

Below is a screenshot of the exchange rules for fill prices:

Exchange rules for fill prices

For the comprehensive US market data, we follow the above rules and do not update the fractional share fill price to the pre-market/after-hours last sale price. Therefore, the discrepancy between the most recent fill price and the displayed pre-market/after-hours last sale price is normal.

Furthermore, different brokers may apply this rule inconsistently. Thus, the latest pre-market and after-hours fill prices may vary across trading platforms. You can go to the specific execution details page to check the price of the last non-fractional share fill.

Key takeaways:

• Trading hours (CST)
  • EDT:
    • Pre-market: 16:00–21:30
    • After-hours: 04:00–08:00
  • EST:
    • Pre-market: 17:00–22:30
    • After-hours: 05:00–09:00
• Characteristics of price mechanism
  • No unified official price: The pre-market and after-hours prices are updated by each broker, not published uniformly by exchanges.
  • Fractional share rules: Fractional share fill prices do not update the high, low, or last sale prices, so the latest fill price might differ from the displayed price.
  • Platform differences: Since brokers may interpret rules differently, pre-market and after-hours prices displayed can vary across platforms.

Disclosures

This article is for reference only and does not constitute any investment advice.