mtime_cache creates a cache of file modification times, based on a glob pattern. If a cache exists it updates unchanged files (unchanged based on MD5 hash) with the time from the cache. This is useful if you cache your build artifacts for a build process which detects changes based on source modification time (such as most C or C++ build systems) on a continuous integration service (such as Travis CI), which clones the repo for every build. When the repo is cloned, all source files have a modification time equal to the current time, making the cached build artifacts (for example .o files) obsolete. mtime_cache allows you to cache the modification times of the files, enabling a minimal rebuild for each clone.