As described in the prospectus, the NAV is calculated by a standard methodology. Typically this is set when the ETF’s underlying securities’ markets close. The closing market price however is the recorded last price of the trading day. This price tends to reflect the supply and demand at the time of market close. Any ETF can trade as late as 4:00 p.m. ET during the normal trading day session, however, in light volume ETFs, its last trade can occur much earlier in the day.