How can I update a setting based on the build?

Question

I have developed a HTTP resource, that uses the Last-Modified header, and it should reflect any modifications made to the application. I would like to update this field to tell the build date, or the last application update date. Here is some solutions I have thought of:

1. Use a build script to change a DateTime setting whenever the application is built.

2. Get the latest modification time of all files in the application folder

What approach seems more interesting? Do you have any suggestion?