13.12 uses different type for xmlelement.
Packet is a type of record #xmlel, so you need to insert new element to Packet#xmlel.children.
on_filter_packet({From, To, #xmlel{ children=OldChildren } = Packet}=Input) ->
...
TimeElem = #xmlel{ name = <<"time">>,
children =
[{xmlcdata, <<"testtime">>}]},
NPacket = Packet#xmlel{ children = [TimeElem|OldChildren] },
...
Not tested, but will work.