You need to attach a Timer Elapsed event like:
CooldownTracker = new Timer();
CooldownTracker.Elapsed += CooldownTracker_Elapsed; //HERE
CooldownTracker.Interval = 1000;
CooldownTracker.Start();
and then the event:
void CooldownTracker_Elapsed(object sender, ElapsedEventArgs e)
{
DecreaseCooldown(yourList);
}