How to prevent /tmp files from being deleted at reboot

[MI <mi.lists.debian@alma.ch>]

In Debian Jessie, systemd ignores the TMPTIME variable in /etc/default/rcS and just 
blindly deletes everything on every reboot.

A bug has been filed about it:  "#795269 TMPTIME not honored anymore"
( https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=795269 )

But I tried the suggested solution, and the files in /tmp still get deleted at reboot.

What I currently have which doesn't work:

   $ egrep -v '^(#|$)' /etc/tmpfiles.d/tmp.conf
   D /tmp 1777 root root 30d
   x /tmp/systemd-private-%b-*
   X /tmp/systemd-private-%b-*/tmp
   x /var/tmp/systemd-private-%b-*
   X /var/tmp/systemd-private-%b-*/tmp


Would someone know what I can do so that only files older than some number of days 
get deleted?