If I filed for an extension for my 2017 income tax return, do I have more time to make a catchup contribution to my employer’s 401(k) plan?

Employee contributions to 401(k) plans are done on an annual basis. There is no period after the end of the year to complete contributions. So unlike IRA contributions that can be made up to the filing deadline of the return, or SEP contributions by a self-employed individual up to the extended due date of the return, salary reduction contributions to a 401(k) plan must be completed by the end of the year to which they relate.