Pub. Contract Code, § 20439

He may extend the time fixed for completion of the work from time to time, under the direction of the legislative body. All applications for extensions of time, if in writing, must be filed in the office of the clerk before the expiration of the original time fixed in the contract, or of the time granted by extension, as the case may be. An extension of time may be granted by the legislative body after the expiration of the time originally fixed in the contract or as previously extended, and the extension so granted shall be deemed to commence and be effective from the date of such expiration. Any extension of time shall not release the sureties upon any bond required under this division.