code: func (t *TombstoneGC) expireTime(expires time.Time) {
  	// This is careful to take the lock only while we are fetching the index
  	// since the channel write might get blocked for reasons that could also
  	// need to hint GC (see #3700).
  	if index := t.purgeBin(expires); index > 0 {
  		t.expireCh <- index
  	}
  }
docs: // expireTime is used to expire the entries at the given time.