And once the asset is sold, that's a taxable event.
1. Assumes the asset in question is publicly traded.
2. Assumes the publicly traded asset has a non trivial amount of trade volume 3. Assumes asset price is relatively stable, moving in a narrow band along a clear trend-line
4. Assumes you have defined the price from the stock information (last trade before close. Daily average, etc)
5. Assumes holder's position is small enough not to affect stock price were they to sell.
And stocks are the easiest to do this with!
Look at the Trump vs NY court case for the value of his house in FL. Unlike the valuation imposed by government fiat, the valuation was agreed to freely by the parties. The courts found it excessive (and it might be) and proposed a valuation so ridiculously low it alone gives Trump grounds to appeal that the judge is either incompetent on the matter or has a personal bias and should anyway have recused himself.
This is just wrong. The very low valuation was not proposed by the NY court, but by the Palm Beach County tax appraiser. This is because the property is deeded for use as a social club rather than a private residence (a condition of sale when Trump purchased it iirc, and one which affects future disposal of the property) and as a commercial entity the value is appraised as a multiple of business income.