Safe Harbor Plans

Can a plan impose a service condition that is longer than 12 months for receive safe harbor matching contributions?  

Nope

No.

However, when combined with certain entry dates, it is possible that an employee might not become eligible for safe harbor contributions for almost 18 months after their date of hire.

Thanks for the responses.  I did not see this in the 401(k) statute or regulations.  Did I miss it or is there another source?

401(k)(2)(D) says that you can not impose a service requirement longer than the maximum under 410(a), without regard to the 2-year eligibility rule.

1.401(k)-1(b)(1)(ii) says that a 401(k) plan (other than a SIMPLE) must satisfy either the ADP test, or the safe harbor provisions. 1.401(k)-1(b)(4)(iv)(B) prohibits the use of restructuring to satisfy section 401(k). Thus, the entire plan must satisfy either the ADP test or the safe harbor provisions; you could not, for example, apply the ADP test only to employees with less than two years of service and rely on the safe harbor for employees with two or more years of service.

1.401(k)-3(h)(3) specifically calls out the ability to use the option to disaggregate otherwise excludable employees as a means of satisfying section 401(k) for plans that require 1 year of service for safe harbor contributions, but allow early participation for deferrals.