Why the Billionaires’ Tax Matters to You, Too - WSJ

This (calculating unrealized gains) so-called mark-to-market approach is used by companies, especially financial firms, in reports to investors.

Most famously by the good folks at Enron!

“The income-tax deferral on unrealized gains is unfair. It allows the wealthiest Americans to avoid tax during a time of increasing concern with inequality. Most Americans can’t take advantage of these strategies,” says Ari Glogower, a professor at Ohio State University’s law school.

That may be because over 40% Americans pay $0 in income tax.