@mfilser thanks for posting this!
in general, I believe we should operate as close as possible to what system tools would do, in this case mv. so when I read the issue you described in #720, it made sense to me that we should do as you say, i.e., preserve the original modification date of target directories when they move to a different file system (partition). but when I looked at this PR it took me time to figure out why you've changed the modification date of the source folders and now I see - we use cached files so by doing that, the date of the source folder is retrieved before it changes later on when removing its children. so let me post an alternative way to achieve this and then see if that also resolves the issue