exceptions.

Bonds shall not be issued or sold by a school district, county or municipality after the expiration of four years from the date of the election authorizing the issue, except for the purpose of refunding previous bond issues or in payment of judgments. The bonds may be sold to the United States or to the state in any case in which the state or the United States has made an offer to purchase the bonds and the offer was accepted prior to the expiration of the four-year period. Any period of time when the validity of bonds or the election therefor is in litigation shall be excluded from the four-year period.