Unless an extension of time is granted, returns made on the basis of the calendar year must be filed on or before April 15 following the close of the calendar year, and returns made on a basis of a fiscal year must be filed on or before the 15th day of the fourth month following the close of the fiscal year. If an extension is granted, returns must be filed on or before the expiration of the period of extension. If the last day for filing returns falls on a Saturday, Sunday or other legal holiday, returns may be filed on the day following without penalty. Returns filed by mail are deemed to have been filed as of the date they are placed in the United States mail. If returns are so posted, properly addressed and postage prepaid, no penalty will attach should the returns not reach the Franchise Tax Board's office until after the due date thereof. However, see Section 18567 for rules pertaining to taxpayers residing or traveling abroad.