Use floor of mtime instead of rounding for thumb property
This seems to be the correct thing to do, at least more in line with what at least one other implementation does. Anything using gnome-desktop  effectively does the same, as gnome_desktop_thumbnail_is_valid applies atol(3) to mtime for comparison and time_t on GNU/Linux is a signed int.  https://git.gnome.org/browse/gnome-desktop/ * lisp/image-dired.el (image-dired-create-thumb): Use floor here.
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