Using API v2, you can submit and fill orders during pre-market and after-hours. Full Extended hours trading has specific risks due to the less liquidity. Please read through Alpaca's Extended Hours Trading Risk Disclosure for more details.
Currently, we support full extended hours:
Pre-market: 4:00am - 9:30am ET
After-hours: 4:00pm - 8:00pm ET
For more information please read the Alpaca Docs.