Negotiations between a public employer and the bargaining representative in a unit of uniformed personnel shall be commenced at least five months prior to the submission of the budget to the legislative body of the public employer. If no agreement has been reached sixty days after the commencement of such negotiations then, at any time thereafter, either party may declare that an impasse exists and may submit the dispute to the commission for mediation, with or without the concurrence of the other party. The commission shall appoint a mediator, who shall forthwith meet with the representatives of the parties, either jointly or separately, and shall take such other steps as he or she may deem appropriate in order to persuade the parties to resolve their differences and effect an agreement: PROVIDED, That a mediator does not have a power of compulsion.