When does a port get assigned if SQL Server is set to TCP Dynamic Ports

I'm trying to determine when a dynamic port is assigned to SQL Server. If the TCP/IP Properties have the TCP Dynamic Ports enabled, does that port get assigned at the time the Server is installed or does it get assigned each time the instance is started. Is it therefore possible that a new port will get assigned each time the instance is restarted? I'm mostly curious about the 2008 through 2014 versions.