feat: add ability to transform js date back to excel

index.d.ts

@@ -2,5 +2,6 @@
 // https://github.com/oleg-koval/excel-date-to-js/issues/16
 
 declare module "excel-date-to-js" {
-  export function getJsDateFromExcel(excelDate: number): Date
+  export function getJsDateFromExcel(excelDate: string | number): Date
+  export function getExcelDateFromJs(date: Date): number
 }

index.js

@@ -1,3 +1,9 @@
+const SECONDS_IN_DAY = 24 * 60 * 60;
+const MISSING_LEAP_YEAR_DAY = SECONDS_IN_DAY * 1000;
+const MAGIC_NUMBER_OF_DAYS = (25567 + 2);
+
+const isDate = date => Object.prototype.toString.call(date) === "[object Date]" && !isNaN(date.getTime())
+
 /**
  * JavaScript dates can be constructed by passing milliseconds
  * since the Unix epoch (January 1, 1970) example: new Date(12312512312);
@@ -14,18 +20,21 @@ module.exports.getJsDateFromExcel = (excelDate) => {
     throw new Error('wrong input format')
   }
 
-  const secondsInDay = 24 * 60 * 60;
-  const missingLeapYearDay = secondsInDay * 1000;
-  const delta = excelDate - (25567 + 2);
-  const parsed = delta * missingLeapYearDay;
+  const delta = excelDate - MAGIC_NUMBER_OF_DAYS;
+  const parsed = delta * MISSING_LEAP_YEAR_DAY;
   const date = new Date(parsed)
 
-  if (Object.prototype.toString.call(date) === "[object Date]" ) {
-    if (isNaN(date.getTime())) {
+  if (!isDate(date)) {
       throw new Error('wrong excel date input')
-    }
-    else {
-      return date
-    }
   }
+
+  return date
+}
+
+module.exports.getExcelDateFromJs = (date) => {
+  if (!isDate(date)) {
+      throw new Error('wrong input format')
+  }
+
+  return (date.getTime() / MISSING_LEAP_YEAR_DAY) + MAGIC_NUMBER_OF_DAYS
 }

test/index.js

@@ -1,13 +1,20 @@
 const test = require('tape');
-const { getJsDateFromExcel } = require('../index');
+const { getJsDateFromExcel, getExcelDateFromJs } = require('../index');
 
-test('should convert Excel date to JS date', function (t) {
+test('should convert Excel date to JS date (number)', function (t) {
   const actual = getJsDateFromExcel(42510).toISOString();
   const expected = new Date('2016-05-20T00:00:00.000Z').toISOString();
   t.equal(actual, expected);
   t.end()
 });
 
+test('should convert Excel date to JS date (string)', function (t) {
+  const actual = getJsDateFromExcel('42510').toISOString();
+  const expected = new Date('2016-05-20T00:00:00.000Z').toISOString();
+  t.equal(actual, expected);
+  t.end()
+});
+
 test('should convert return an error on wrong input', function (t) {
   try {
     const actual = getJsDateFromExcel('text');
@@ -25,3 +32,32 @@ test('should convert return an error on wrong excel date', function (t) {
   }
   t.end()
 });
+
+test('should convert JS date to Excel date',  (t) => {
+  const date = new Date('2016-05-20T00:00:00.000Z');
+  const expected = 42510;
+
+  const actual = getExcelDateFromJs(date);
+
+  t.equal(actual, expected)
+
+  t.end()
+});
+
+test('should return an error on wrong input while converting JS date (wrong type)', function (t) {
+  try {
+    getExcelDateFromJs('text');
+  } catch ({ message }) {
+    t.equal(message, 'wrong input format')
+    t.end()
+  }
+});
+
+test('should return an error on wrong input while converting JS date (invalid date)', function (t) {
+  try {
+    getExcelDateFromJs(new Date('text'));
+  } catch ({ message }) {
+    t.equal(message, 'wrong input format')
+    t.end()
+  }
+});