After-hours counts as daytrading?

Suppose you buy stocks in during normal trading hours, then, after the close, you sell them after-hours; using an ECN like ArcaEx.

Is this allowed without a daytrading account?

 

That trade would be a day trade. You would have to hold over night to avoid it.

 

definitely a day trade.

 

As long as both opening trade followed by a closing trade (even partly) executions are done on the same day i.e., "pre-regular market hours"+"regular market hours"+"post regular market hours", it is a day trade.