From the passage provided, extract the year Apache Software Foundation was incorporated on.

Context: The Apache Software Foundation /əˈpætʃi/ is an American nonprofit corporation to support a number of open source software projects. The ASF was formed from a group of developers of the Apache HTTP Server, and incorporated on March 25, 1999. As of 2021, it includes approximately 1000 members.

The Apache Software Foundation is a decentralized open source community of developers. The software they produce is distributed under the terms of the Apache License, a permissive open-source license for free and open-source software. The Apache projects are characterized by a collaborative, consensus-based development process and an open and pragmatic software license, which is to say that it allows developers who receive the software freely, to re-distribute it under nonfree terms. Each project is managed by a self-selected team of technical experts who are active contributors to the project. The ASF is a meritocracy, implying that membership of the foundation is granted only to volunteers who have actively contributed to Apache projects. The ASF is considered a second generation open-source organization, in that commercial support is provided without the risk of platform lock-in.

Among the ASF's objectives are: to provide legal protection to volunteers working on Apache projects; to prevent the Apache brand name from being used by other organizations without permission.

The ASF also holds several ApacheCon conferences each year, highlighting Apache projects and related technology.

Answer: Apache Software Foundation was incorporated on March 25, 1999