report_last_modified() returns the last modified date and time of the
report source: local file or remote file. If a local file, the modification
date will be that indicated by the file system; if a remote file, the date
of last update is that provided by HTTP header "last-modified".
MGI updates its reports weekly, every Thursday. However, not all reports are
updated each week. The return value of this function is the closest you will
get to a versioning of MGI report files.