The subtraction is easy and the valuation is not, which is the whole substance of the subject. Rule 2a-4(a) opens by saying that current net asset value means an amount reflecting calculations "made substantially in accordance with the following, with estimates used where necessary or appropriate." A regulation that builds estimation into its own definition is telling you something: this is a considered accounting figure, not the record of a transaction.
The rule's first paragraph is where the judgment sits. Portfolio securities "with respect to which market quotations are readily available shall be valued at current market value, and other securities and assets shall be valued at fair value as determined in good faith by the board of directors of the registered company." So a fund holding large listed shares is mostly reading prices off a screen. A fund holding thinly traded corporate bonds, private credit, restricted stock or securities whose market closed hours before the fund's own valuation time is doing something else, and the responsibility for that something else sits with the board rather than with a market. The practical consequence for an investor is that the reliability of a net asset value varies with what the fund holds, and the funds where it matters most are precisely the ones where it is hardest.
The rest of Rule 2a-4 is a list of timing conventions, and they explain small discrepancies that otherwise look like errors. Changes in portfolio holdings must be reflected no later than the first calculation on the first business day following the trade date, and changes in the number of shares outstanding from distributions, redemptions and repurchases no later than the first calculation on the first business day after the change. Expenses, including any investment advisory fees, are included to the date of calculation, which is why a fund's costs come out of the value continuously rather than arriving as a charge. Dividends receivable are included at ex-dividend or record dates as appropriate, and interest and other income to the date of calculation. There is a de minimis allowance: accrued expenses and income need not be reflected if, netted, they do not amount to as much as one cent per outstanding share.
How often the calculation happens depends on whether shares are redeemable. The SEC states that mutual funds and unit investment trusts generally must calculate net asset value at least once every business day, typically after the major US exchanges close, and that a closed-end fund, whose shares are not redeemable, is not subject to that requirement. The reason is structural rather than administrative. A fund obliged to buy its own shares back needs a defensible price to buy them back at, every day it might be asked. A fund under no such obligation does not.
That distinction also decides what the number means to you. When you buy or redeem a mutual fund you transact at its net asset value, so for that fund the computed figure is the price. An exchange-traded fund or a closed-end fund is bought from and sold to other investors on an exchange, so its market price is set by trading and the net asset value sits alongside as a reference. The two can differ, and how far they can drift apart differs sharply between those two structures. The mechanics of that gap belong with the pages on those funds rather than here.
One naming point saves confusion on a fund statement. Net asset value per share is not the same idea as the market capitalization of a company, and it is not a valuation opinion about whether the fund is cheap. It is a bookkeeping measure of what the fund currently owns net of what it owes, restated per share, and it moves for two quite different reasons: because the holdings changed in value, and because money came in or went out.
