Tag updates

[Freebaser]

How could I update a tag every second? I want it to return the time left on a buff.

Here's what I have so far.

oUF.Tags["[rnwTime]"] = function(u)
  local name, _,_,_,_,_, expirationTime, fromwho,_ = UnitAura(u, L["Renew"])
  if not (fromwho == "player") then return end
  local spellTimer = "|cffffff00"..format("%.0f",-1*(GetTime()-expirationTime)).."|r"
  return spellTimer end
oUF.TagEvents["[rnwTime]"] = "UNIT_AURA"