What are the deadlines for making employee and employer contributions to an Individual 401k, and does a filing extension make a difference?

[ee-ea]

See this from the IRS

https://www.irs.gov/newsroom/tax-time-guide-saving-for-retirement-ira-contributions-for-2021-can-be-...

[dmertz · Accepted solution]

The deadline for electing to make an employee contribution to an individual 401(k) is December 31 and the employee contribution should be made as soon as is practical after that.  The deadline for making an employer contribution is the due date of your tax return, including extensions.