The written statements of qualification and economic interests shall be filed by (i) primary candidates not later than the filing deadline for the primary, (ii) all other candidates for city and town offices to be filled at a May general election by 7:00 p.m. on the first Tuesday in March, (iii) candidates in special elections by the time of qualifying as a candidate, and (iv) all other candidates by 7:00 p.m. on the third Tuesday in June. A statement shall be deemed to be timely filed if it is mailed postage prepaid to the appropriate office by registered or certified mail and if the official receipt therefor, which shall be exhibited on demand, shows mailing within the prescribed time limits. The State Board may grant an extension of any deadline for filing either or both written statements and shall notify all candidates who have not filed their statements of the extension. Any extension shall be granted for a fixed period of time of ten days from the date of the mailing of the notice of the extension.