Qualified plans must be established by the last day of your fiscal year if contributions are to be made for that year. If you have a calendar year end 12/31/2018, you need to establish the plan on that date for 2018 contributions. 401(k) deferrals must be made no later than 30 days after which the contributions are received. For 12/31/2018 that deadline would be 30 days later. SEP and SIMPLE IRA contributions may be made by the company tax deadline plus extensions, except for defaults, which follow the above rules. IRA contributions must be made by April 15 with no extensions.