Dates with timezones

README.md

@@ -63,6 +63,12 @@ const options = {
    * defaults to false
    */
   dotsForObjectNotation: false,
+
+  /**
+   * display dates in local timezone, with the timezone offset displayed.
+   * otherwise, all dates will be converted to the UTC timezone
+   */
+   dateWithTimezone: false,
 };
 
 const formData = serialize(

src/index.js

@@ -39,6 +39,15 @@ function isFile(value, isReactNative) {
   );
 }
 
+function toTzIsoString(date) {
+    const off    = date.getTimezoneOffset();
+    const absoff = Math.abs(off);
+    return (new Date(date.getTime() - off*60*1000).toISOString().substr(0,23) +
+            (off > 0 ? '-' : '+') + 
+            Math.floor(absoff / 60).toFixed(0).padStart(2,'0') + ':' + 
+            (absoff % 60).toString().padStart(2,'0'))
+}
+
 function initCfg(value) {
   return isUndefined(value) ? false : value;
 }
@@ -53,6 +62,7 @@ function serialize(obj, cfg, fd, pre) {
   cfg.allowEmptyArrays = initCfg(cfg.allowEmptyArrays);
   cfg.noFilesWithArrayNotation = initCfg(cfg.noFilesWithArrayNotation);
   cfg.dotsForObjectNotation = initCfg(cfg.dotsForObjectNotation);
+  cfg.dateWithTimezone = initCfg(cfg.dateWithTimezone);
 
   const isReactNative = typeof fd.getParts === 'function';
 
@@ -83,7 +93,11 @@ function serialize(obj, cfg, fd, pre) {
       fd.append(pre + '[]', '');
     }
   } else if (isDate(obj)) {
-    fd.append(pre, obj.toISOString());
+    if (cfg.dateWithTimezone) {
+      fd.append(pre, toTzIsoString(obj));
+    } else {
+      fd.append(pre, obj.toISOString());
+    }
   } else if (isObject(obj) && !isBlob(obj, isReactNative)) {
     Object.keys(obj).forEach((prop) => {
       const value = obj[prop];

src/index.test.js

@@ -291,6 +291,28 @@ test('Date', () => {
   expect(formData.get('foo')).toBe(foo.toISOString());
 });
 
+test('Date with timezone', () => {
+  const toTzIsoString = (date) => {
+    const off    = date.getTimezoneOffset();
+    const absoff = Math.abs(off);
+    return (new Date(date.getTime() - off*60*1000).toISOString().substr(0,23) +
+            (off > 0 ? '-' : '+') + 
+            Math.floor(absoff / 60).toFixed(0).padStart(2,'0') + ':' + 
+            (absoff % 60).toString().padStart(2,'0'))
+  }
+  const foo = new Date(2000, 0, 1, 1, 1, 1);
+  const formData = serialize({
+    foo,
+  },
+  {
+    dateWithTimezone: true,
+  });
+
+  expect(formData.append).toHaveBeenCalledTimes(1);
+  expect(formData.append).toHaveBeenCalledWith('foo', toTzIsoString(foo));
+  expect(formData.get('foo')).toBe(toTzIsoString(foo));
+});
+
 test('File', () => {
   const foo = new File([], '');
   const formData = serialize({

types.d.ts

@@ -5,6 +5,7 @@ export type Options = {
   allowEmptyArrays?: boolean;
   noFilesWithArrayNotation?: boolean;
   dotsForObjectNotation?: boolean;
+  dateWithTimezone?: boolean;
 };
 
 export function serialize<T = {}>(