TFS 0.X Temporary skill

[jestem pro]

Hello, I have a little problem with onLogin function.

Generally I wanna add player for example 5 shield skill. There is a part in "actions", which add the skill for 30 sec.

But when player relog the skill is being reseted. It's made by createConditionObject:

local storage = 12347

local checkingskill

local condition = createConditionObject(CONDITION_ATTRIBUTES)
setConditionParam(condition, CONDITION_PARAM_SKILL_SHIELD, 5)
setConditionParam(condition, CONDITION_PARAM_TICKS, checkingskill * 1000)
setConditionParam(condition, CONDITION_PARAM_BUFF, true)

function onLogin(cid)

  if getCreatureStorage(cid, storage) > os.time() then
 
          checkingskill = getCreatureStorage(cid, storage) - os.time()
         doAddCondition(cid, condition) 
     
   end
    return true
end

The explanation:
When player use an item it adds him +5 shield skill for 30 sec. Then if player relog the skill should be added for as many seconds as it was left.

Any suggestions how we can solve it or make in other way?