code: func dateWrapper(vals []Value, enh *EvalNodeHelper, f func(time.Time) float64) Vector {
  	if len(vals) == 0 {
  		return append(enh.out,
  			Sample{
  				Metric: labels.Labels{},
  				Point:  Point{V: f(time.Unix(enh.ts/1000, 0).UTC())},
  			})
  	}
  
  	for _, el := range vals[0].(Vector) {
  		t := time.Unix(int64(el.V), 0).UTC()
  		enh.out = append(enh.out, Sample{
  			Metric: enh.dropMetricName(el.Metric),
  			Point:  Point{V: f(t)},
  		})
  	}
  	return enh.out
  }
docs: // Common code for date related functions.