[UI] Sorting "Last Seen" on "Dashboard" sorts by string, not by timestamp

When trying to sort nodes on the "Dashboard" "Network Discovery" page by "Last Seen", the UI sorts by string (alphabetic sorting, "Fri, 16 Feb. 2018 17:41:33", then "Mon, 15 Jan. 2018 14:29:07") instead of by actual last seen numeric timestamp ("Mon, 15 Jan. 2018 14:29:07" then "Fri, 16 Feb. 2018 17:41:33")

Including MaaS version:
root@maas:~# dpkg -l '*maas*'
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name Version Architecture Description
+++-=================================================-=============================-=============================-=======================================================================================================
ii maas 2.3.0-6434-gd354690-0ubuntu1~ all "Metal as a Service" is a physical cloud and IPAM
ii maas-cli 2.3.0-6434-gd354690-0ubuntu1~ all MAAS client and command-line interface
un maas-cluster-controller <none> <none> (no description available)
ii maas-common 2.3.0-6434-gd354690-0ubuntu1~ all MAAS server common files
ii maas-dhcp 2.3.0-6434-gd354690-0ubuntu1~ all MAAS DHCP server
ii maas-dns 2.3.0-6434-gd354690-0ubuntu1~ all MAAS DNS server
ii maas-proxy 2.3.0-6434-gd354690-0ubuntu1~ all MAAS Caching Proxy
ii maas-rack-controller 2.3.0-6434-gd354690-0ubuntu1~ all Rack Controller for MAAS
ii maas-region-api 2.3.0-6434-gd354690-0ubuntu1~ all Region controller API service for MAAS
ii maas-region-controller 2.3.0-6434-gd354690-0ubuntu1~ all Region Controller for MAAS
un maas-region-controller-min <none> <none> (no description available)
un python-django-maas <none> <none> (no description available)
un python-maas-client <none> <none> (no description available)
un python-maas-provisioningserver <none> <none> (no description available)
ii python3-django-maas 2.3.0-6434-gd354690-0ubuntu1~ all MAAS server Django web framework (Python 3)
ii python3-maas-client 2.3.0-6434-gd354690-0ubuntu1~ all MAAS python API client (Python 3)
ii python3-maas-provisioningserver 2.3.0-6434-gd354690-0ubuntu1~ all MAAS server provisioning libraries (Python 3)