func (win *Window) SetLeft(marker time.Time) {
	win.leftMarker = marker
}
SetLeft defines the left marker for the Window
package ais
import (
	"bytes"
	"fmt"
	"time"
)
type Window struct {
	leftMarker, rightMarker time.Time
	timeIndex               int
	width                   time.Duration
	Data                    map[uint64]*Record
}
func NewWindow(rs *RecordSet, width time.Duration) (*Window, error) {
	win := new(Window)
	timeIndex, ok := rs.Headers().Contains("BaseDateTime")
	if !ok {
		return nil, fmt.Errorf("newwindow: headers does not contain BaseDateTime")
	}
	win.SetIndex(timeIndex)
	rec, err := rs.readFirst()
	if err != nil {
		return nil, fmt.Errorf("newwindow: %v", err)
	}
	t, err := time.Parse(TimeLayout, (*rec)[timeIndex])
	if err != nil {
		return nil, fmt.Errorf("newwindow: %v", err)
	}
	win.SetLeft(t)
	win.SetWidth(width)
	win.SetRight(win.Left().Add(win.Width()))
	return win, nil
}
func (win *Window) Left() time.Time { return win.leftMarker }