The atime of a file is supposed to change whenever the data blocks of a file are being read. This can be costly performancewise when an application regularly accesses a very large number of files or directories. Some Unix filesystems can be mounted with atime updates disabled to increase the performance of such applications; USENET news spools are a common example. On such filesystems this function will be useless.
In most Unix filesystems, a file is considered changed when its inode data is changed; that is, when the permissions, owner, group, or other metadata from the inode is updated. Note also that in some Unix texts the ctime of a file is referred to as being the creation time of the file. This is wrong. There is no creation time for Unix files in most Unix filesystems.
This function returns the time when the data blocks of a file were being written to, that is, the time when the content of the file was changed.