Solo 401(k)

Not IRA specific but hoping someone can provide guidance

What is/was the deadline for a sole proprietor/Schedule C to make 2023 Roth salary deferral contributions and employer contributions (profit sharing) to a previously established (in 2022) Solo 401(k)?

My research is giving me conflicting answers

thank you

The deferral election had to be made by 12/31/2023 and the deposits for both sources made by the 2023 tax filing deadline plus extensions