excel4node Dates

// Uses excel4node version 1.x.x

var xl = require('excel4node');

var wb = new xl.Workbook({
    dateFormat: 'm/d/yy hh:mm:ss'
});

var ws = wb.addWorksheet('Dates');

ws.cell(1, 1).date(new Date());
ws.cell(2, 1).date(new Date()).style({ numberFormat: 'yyyy-mm-dd' });

ws.column(1).setWidth(20);

wb.write('DateSample.xlsx');

mm stands for minutes or months?

mm for minutes with two digits eg:- 04, 10, 09 even if you have a single number it will represent a double-digit.
MM Stands for double-digit to represent the month.