NAV is struck at a valuation point, usually the close of the primary market. Holdings are marked at last traded or evaluated prices, accrued income is added, and fees payable are deducted before dividing by the share count.
The number is only as good as the marks behind it. For exchange-traded equities NAV is close to exact; for high-yield bonds, loans or property it relies on evaluated pricing or appraisals that may lag real transaction levels by weeks. Stale marks are why some funds appear less volatile than their assets really are.
Where the fund's holdings trade in a time zone that has already closed, the NAV can be stale by hours, which was the mechanism behind historic market-timing abuses and the reason fair-value pricing adjustments now exist.
Related: mutual-fund, closed-end-fund-discount, open-end-fund, mark-to-market, etf-arbitrage, liquidity