Treasury Secretary Janet Yellen suggested in remarks before a Senate panel that if Congress were to pass a capital-gains tax hike effective starting in April 2021, that wouldn’t count as a retroactive increase.

“I don’t see a prospective change in rules pertaining to the taxation of future realization of capital gains as being a retroactive feature,” Yellen told the Senate Finance Committee on Wednesday, when asked about the Biden administration’s tax proposals.

President Joe Biden’s proposal to raise the capital-gains tax rate to 39.6% from 20% for those earning $1 million or more was first announced April 28, as part of the administration’s American Families Plan.

Yellen’s comment then suggests that the White House is pushing for the capital-gains increase to be effective before the date a law passes Congress — and it could take months more of negotiations before a bill is finalized. It also raises questions for investors who are wondering if it’s already too late to head off a tax hike.

Debate over the start date of any capital-gains tax changes is likely to be a source of tension as the issue is debated in Congress. Some Democrats, including Senator Mark Warner of Virginia, are expressing doubts about the White House’s proposal and Democratic aides for the House Ways and Means Committee have said they don’t prefer to do retroactive tax increases.

Rising concern

The uncertainty about when higher rates for the wealthy could kick in has caused much consternation for investors and their tax advisers, who are determining whether to speed up sales of highly appreciated corporate stocks, real estate and businesses before tax rates could roughly double. The Biden administration has suggested that they are pushing for most of the other tax increases, if approved by Congress, to begin in 2022.

Capital-gains taxes are only paid when an asset is sold, giving lots of flexibility to investors to pay taxes, or defer them indefinitely. Democrats have criticized this treatment, saying it allows the wealthy to pay far lower tax rates than middle class households, who rely more on regular income than on wealth gains.

Biden’s proposal to tax capital-gains income at the same rates as wages and salaries for high earners would be one of the biggest changes to investment taxation in roughly a century. Biden also wants to end a tax break that wipes away taxable capital gains at death, allowing families to escape taxes on appreciated assets when they’re inherited.