If you have a project using Doxygen for documentation and Git for source control management you may use this trick in doxygen.conf:

FILE_VERSION_FILTER = "/bin/sh -c 'git log --pretty=\"format:%ci\" -1 \"${1}\" || echo no git'"

This will show date of the last commit in the header:

simple

You can give even more useful information using git’s pretty formats:

FILE_VERSION_FILTER = "/bin/sh -c 'git log --pretty=\"format:%ci, author:%aN <%aE>, commit:%h\" -1 \"${1}\" || echo no git'"

This looks like:

detailed

Note this may vastly increase runtime of Doxygen if you have lots of files to process, but I think it is a nice way to give information to project users.