-dit(bf(-u, --update)) This forces rsync to skip any files for which the
-destination file already exists and has a date later than the source
-file.
+dit(bf(-u, --update)) This forces rsync to skip any files which exist on
+the destination and have a modified time that is newer than the source
+file. (If an existing destination file has a modify time equal to the
+source file's, it will be updated if the sizes are different.)