For example, if your traditional IRA balance was zero on December 31 2020 and you made a nondeductible traditional IRA contribution for 2020 in January 2021, this traditional IRA contribution would go on your 2020 Form 8606.
Depending on your other Roth conversions, TurboTax may or may not treat the basis resulting from this contribution as being applicable to any 2020 Roth conversions through the use of Worksheet 1-1 of IRS Pub 590-B, which is why I chose to be a bit vague with respect to this aspect of the reporting. TurboTax seems to be rather liberal with the use of this worksheet. If TurboTax does use the worksheet in this case, the basis from the contribution for 2020 made in 2021 might be used to reduce the taxable amount of the 2020 distributions and the basis so used would therefore not be carried forward to be used to reduce the taxable amounts of Roth conversions done in 2020 or later. It's not that any basis is lost or you end up paying taxes twice on any money, it just affects which year's tax return shows the taxable amounts.
