Your system, once
screen detaches, destroys the
pts, and for some reason recreates it, if I understand correctly how
udev is handling things on your system.
udev is the subsystem that controls the creation and destruction of devices in /dev, which is a dynamically generated filesystem.
pts creation/destruction is handled by
ptmx, which is used to create pseudo-terminal master/slave pairs.
pts/* is the slave of the respective
PTM, or pseudo-terminal master. As such, any permissions modifications that you see are a direct result of the destruction and creation of said device nodes, rather than modification. As for the date of the file, since the device nodes are clones, it's likely that the original used to create these nodes had a creation date of the time that you see in your
man ptmx -- Describes how ptmx creates new pts pseudo terminal device nodes.
What I don't understand is why there is a difference between how your system behaves versus how mine behaves with regard to
/dev/pts/*. I do not experience perceived perms changes to devices; They either disappear entirely which is as it should be, or the perms do not change regardless of my actions (e.g., detaching screen, the device stays, and does not get destroyed/recreated.). Not only that, but the dates associated with my newly created
pts/* devices are the current date.
One possibility is that the VPS you're using has something to do with this behavior. For example, I can't perform a dist-upgrade on my VPS, since the system they utilize only allows for one kernel version, the one they've hacked and put in place. The kind of restrictions that prevent you from updating your own kernel could also impact the functionality of other sub-systems. That's just speculation though, but it would make sense.
It could also just be a difference in how udev is configured.
Revision 3, with a lot of help from Gilles. ;)