In that vein, I played a little with the monthExt, QuarterExt, and modified as below. It appears to work.

{
  "name": "monthExt",
  "update": "[data('xExt')[0]['s']-oneDay ,data('xExt')[0]['s'] + ((ganttWidth-minDayBandwidth)/minDayBandwidth)*2*oneDay]"
},
{
  "name": "quarterExt",
  "update": "[data('xExt')[0]['s']-oneDay, data('xExt')[0]['s'] + ((ganttWidth-minMonthBandwidth)/minMonthBandwidth)*1.5*oneDay]"
},