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