In some contested divorce cases it is necessary for the parties to agree or the court to determine the date an asset should be valued. The choices are (for marital assets) generally either the date the divorce starts or at the time of trial (or near conclusion if settled). To make this concept more complex is that different assets within the same divorce case may have different valuation dates and then of course exceptions can be made as well to prevent an unfair or unjust result. All of this is part of the law of equitable distribution.
The basic concept is that if an asset is "active" the asset will be valued as of the date when the Summons was filed. If the asset is "passive" the value will be set near the end of the case.
Examples of a "passive" asset are items of art, bank accounts that accrue interest/dividends without involvement of either party (perhaps they use a financial planner or the stock market determines the value as it fluctuates) and real estate.
The classic example of an "active" asset is the operation of a business after the divorce has begun and the change in value due to the "sweat labor" of the spouse to grow the business. An owner that develops new products and new customers after the divorce may be able to keep the resulting increase in value of the business separate.
If this seems simple then perhaps the unusual cases are more challenging such as when an unforeseen event completely changes the value of the asset after the divorce has started. For example a business is doing great and making lots of money and then during the divorce the main customer of that business files bankruptcy and doesn't pay the invoices, or a new product is launched by a competitor and half the customer base is lost. Imagine a trendy restaurant that is filled every night and then one weekend there are cases of food poisoning and now the restaurant is empty every night (this happened near where I live!) What if a tariff is placed on a product and customers can't afford to buy that product (such as an imported car) and the dealership volume is cut in half? Clearly the working "owner" spouse will not want to use a date of divorce commencement value under such new unforseen circumstances.
It is clear that the passage of time during a divorce can have a huge impact on valuation issues. I have also seen situations where a large medical corporation decides it wants to purchase the medical practice of five doctors and the negotiations begin after the divorce is filed and a deal is reached to sell the practice for a premium that did not exist when the medical practice was valued as of the date of divorce. In addition the divorcing doctor was only a 15% partner and did not negotiate the deal, had no control of the acceptance or rejection of the offer and so the doctor states that the increase in value of his interest is not due to any effort on his part but likewise only exists because of the other larger partners in the practice. How should the windfall be distributed?
Valuation issues can be extremely complicated and it is critical to have a highly experienced matrimonial attorney represent you once the issues are not the "run of the mill" basic fact patterns.