Your purchase has to settle by that date. It used to be 3 days to settle, but they changed it to 2 days a few years ago.
Washington D.C., March 22, 2017 —
The Securities and Exchange Commission today adopted an amendment to shorten by one business day the standard settlement cycle for most broker-dealer securities transactions. Currently, the standard settlement cycle for these transactions is three business days, known as T+3. The amended rule shortens the settlement cycle to two business days, T+2.
The amended rule is designed to enhance efficiency, reduce risk, and ensure a coordinated and expeditious transition by market participants to a shortened standard settlement cycle.