设置时间格式的单元格

[http://poi.apache.org/spreadsheet/quick-guide.html#CreateDateCells]

CellStyle cellStyle = wb.createCellStyle();
CreationHelper createHelper = wb.getCreationHelper();
cellStyle.setDataFormat(
createHelper.createDataFormat().getFormat("MM/dd/yyyy"));
cell = row.createCell(1);
cell.setCellValue(new Date());
cell.setCellStyle(cellStyle);

有奇效。。。另外将localdate转换为date的方式：
Date.from(a.getStartDate().atStartOfDay().atZone(ZoneId.systemDefault()).toInstant())