§ 100. Mandatory referendum. When a resolution of the board of supervisors is subject to a mandatory referendum, such board shall by resolution cause such resolution to be submitted to a vote of the qualified electors of the county for their approval or disapproval at a general or special election. Except as otherwise provided by law, the resolution shall fix a date not less than forty-five days after its adoption on which the proposition is to be submitted at such general or special election.