code: func (h *Head) initTime(t int64) (initialized bool) {
  	if !atomic.CompareAndSwapInt64(&h.minTime, math.MaxInt64, t) {
  		return false
  	}
  	// Ensure that max time is initialized to at least the min time we just set.
  	// Concurrent appenders may already have set it to a higher value.
  	atomic.CompareAndSwapInt64(&h.maxTime, math.MinInt64, t)
  
  	return true
  }
docs: // initTime initializes a head with the first timestamp. This only needs to be called
  // for a completely fresh head with an empty WAL.
  // Returns true if the initialization took an effect.