[Fizzlemizz]

Maybe something along the lines of:

local usable, _ = IsUsableSpell(spellName)
local start, duration = GetSpellCooldown(spellName)
local ignore, gcd = GetSpellCooldown(61304)
if (usable) then
     if (duration == 0 and (start < ignore)) or (duration <= gcd) then --ignore global cooldown
          --show icon
          Frame.endTime = start + duration
     end
else
      if not Frame.endTime or Frame.endTime <= GetTime() then
          Frame:Hide() -- only if it's not still ticking
      end
end