US Markets operate from Monday to Friday, opening at 9:30 AM Eastern Standard Time (EST) and closing at 4:00 PM.

Note - This time window is EST and not Australian Eastern Standard Time (AEST).

You can place orders outside of these times, however they will only be executed when the market opens.

It's also worth noting that the stock market is closed for most major US holidays - you can find the complete list of holidays on the NYSE website.