US Stock Pre-Market and After-Hours Trading Rules

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

1. Pre- & post-market trading session

In addition to the regular trading sessions, US stocks also have extended pre-market and post-market trading sessions. Due to the 1-hour spread between daylight saving time and standard time in the United States, the trading sessions in GMT+8 are adjusted by 1 hour correspondingly. Daylight saving time begins on the second Sunday in March each year, and standard time begins on the first Sunday in November.

2. Pre- & post-market price mechanism

The US stock exchanges do not have a unified official price for the pre-market and post-market prices. However, brokerage firms adopt the last done price to update the pre- & post-market High/Low/Last Done according to the rules.

While the exchanges do not provide official prices for extended trading sessions, official guidelines are available. We offer pre- & post-market prices to US stocks based on the guidelines from official Nasdaq and NYSE documents.

One of the most important rules is: odd lot (including fractional share) transactions do not affect High/Low/Last Done price.

The following is a screenshot from the exchange:

Therefore, it is possible that the last done price is inconsistent with the pre-market and post-market prices.

In addition, different brokers may apply this rule differently, resulting in different pre-market and post-market prices displayed on different platforms. You can check the specific transaction details page to determine the price of the last "non-odd lot transaction".

Key takeaways:

• Trading hours (SGT)
  • 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.