If the US Congress passes major changes to federal income tax laws, such as new income tax rates, what is the soonest that those changes would normally take effect? Would it be the same year, if passage were early enough in the year? How early in the year would it have to be? Or would it normally take effect the following year?
Congress sets the tax rates with their laws. They also pick the date it goes into effect.
It can start far in the future, in a just a few days, immediately, in the recent past, or even last year. There is no standard.
This has caused problems because the IRS has had to scramble to change tax forms and instructions. They have had to change tax tables and withholding calculators.
In December 2017 they made massive changes to the taxes. The law set a date for some of the changes, specifically related to mortgage deductability. The law was passed and signed a few days after the date specified in the law. Some people after they settled on the house or after they had refinanced now were impacted by the new law, which changed their tax situation.
Many times the congress waited until December to address the AMT, which meant that some tax payers didn't know their tax rate until 11.5 months into the tax year. So the effective date was 11.5 months in the past.
One year they waited until January the next year to set the maximum monthly value for the commuter benefit. So it was 12 months in the past.
Going back to the 2017 changes the December passage meant that some people only had a short time to pay some bills, or make some charitable contributions in order to get them in just before the changes would have made them moot. In my county there was a line to pre-pay property tax before the end of December, later it was determined that the pre-payment wasn't tax deductible. So they want 3 days in the past.
My state (Virginia) spent all of 2018 trying determine what changes they needed to make in response to the Federal tax changes, they ended up making no big changes. That meant that the entire year we didn't know what the Virginia income tax rates would be.
There is no standard.