code: function cellOffsetToDayOffset(cellOffset) {
  		var day0 = t.visStart.getDay(); // first date's day of week
  		cellOffset += dayToCellMap[day0]; // normlize cellOffset to beginning-of-week
  		return Math.floor(cellOffset / cellsPerWeek) * 7 // # of days from full weeks
  			+ cellToDayMap[ // # of days from partial last week
  				(cellOffset % cellsPerWeek + cellsPerWeek) % cellsPerWeek // crazy math to handle negative cellOffsets
  			]
  			- day0; // adjustment for beginning-of-week normalization
  	}
docs: cell offset -> day offset