mtimer/system/on_joinplayer.lua

local m = mtimer

minetest.register_on_joinplayer(function(player)
    local meta = player:get_meta()
    for _,def in pairs(m.meta) do
        local current = meta:get_string(def.key)
        if current == '' then meta:set_string(def.key, def.default) end
    end
    meta:set_string('mtimer:session_start', os.time())
end)