Contributions for 401(k) have two deadlines:
Contributions deadline for Employees elective deferrals is last paycheck or 12/31/2017
Total Contribution deadline (Employer non-elective deferrals) is until 4/15/2018 (plus extensions)
https://www.irs.gov/retirement-plans/plan-participant-employee/retirement-topics-401k-and-profit-sha...
