Mix.Project.config_mtime

You're seeing just the function config_mtime, go back to Mix.Project module for more information.
Link to this function

config_mtime()

View Source (since 1.7.0)

Specs

config_mtime() :: posix_mtime when posix_mtime: integer()

Returns the latest modification time from config files.

This function is usually used in compilation tasks to trigger a full recompilation whenever such configuration files change. For this reason, the mtime is cached to avoid file system lookups.