XLSX writing custom date formats

main.js

/* this sample was run in nodejs */
var XLSX = require('xlsx');

/* build up a very simple workbook */
var wb = {
	SSF: XLSX.SSF.get_table(),
	SheetNames: ["Sheet1"],
	Sheets: {
		Sheet1: {
			'!ref': 'A1:A1',
			A1: {
				v: 12345,
				t: 'n'
			}
		}
	}
}

/* converts date to number type (in the future, will be embedded in the writer) */
function datenum(v, date1904) {
	if(date1904) v+=1462;
	var epoch = Date.parse(v);
	return (epoch - new Date(Date.UTC(1899, 11, 30))) / (24 * 60 * 60 * 1000);
}

/* version 0.7.9 does not allocate entries for custom formats, this is what it
   should do (and it will do something like this in a later version) */
function add_custom_format(wb, fmt) {
	var i;
	for(i = 0; i < 0x188; ++i) if(wb.SSF[i] === fmt) return;
	for(i = 0xA4; i < 0x188; ++i) if(!wb.SSF[i]) { wb.SSF[i] = fmt; return; }
}


/* this function performs the required magic */
function assign_date_with_format(wb, sheet, cell, date, fmt) {
	/* create sheet if it does not exist */
	if(wb.SheetNames.indexOf(sheet) === -1) SheetNames.push(sheet);
	if(!wb.Sheets[sheet]) wb.Sheets[sheet] = {};
	if(!wb.Sheets[sheet]['!ref']) wb.Sheets[sheet]['!ref'] = cell + ':' + cell;

	/* update sheet range if the new cell is outside of range */
	var r = XLSX.utils.decode_range(wb.Sheets[sheet]['!ref']);
	var c = XLSX.utils.decode_cell(cell);
	wb.Sheets[sheet]['!ref'] = XLSX.utils.encode_range({
		s: {r: Math.min(r.s.r, c.r), c: Math.min(r.s.c, c.c) },
		e: {r: Math.max(r.e.r, c.r), c: Math.max(r.e.c, c.c) },
	});

	/* create cell if it does not exist */
	if(!wb.Sheets[sheet][cell]) wb.Sheets[sheet][cell] = {t:'n', v:0};

	/* load the format */
	add_custom_format(wb, fmt);

	/* assign the date to the cell */
	wb.Sheets[sheet][cell].t = 'n';
	wb.Sheets[sheet][cell].v = datenum(date, false);

	/* set the format */
	wb.Sheets[sheet][cell].z = fmt;

	/* this call updates the formatted text field (`.w`) */
	XLSX.utils.format_cell(wb.Sheets[sheet][cell]);
}


/* sample format and data */
var fmt = 'yyyy/m';
var dates = [
	new Date(2014, 5, 22),
	new Date(2014, 6, 22),
	new Date(2014, 7, 22),
]
var sheet = 'Sheet1', cell = '';

for(var d = 0; d < dates.length; ++d) {
	cell = XLSX.utils.encode_cell({c:0, r:d});
	assign_date_with_format(wb, sheet, cell, dates[d], fmt);
}

console.log(wb.Sheets.Sheet1);

XLSX.writeFile(wb, 'testfmt.xlsx');