Contributions are entered for the tax year.

Conversions are for the calendar year of conversion.

If you contribute a 2025 nondeducted Trad IRA amount now, and convert it now, that 1099-R and conversion are for 2026. There is no backdating a conversion. The Basis is from tax year 2025.

So, if you didn't convert the full amount, but convert the rest in 2027, it's a 2027 conversion. The year of contribution is meaningless. Basis is what matters, for Backdoor.