diff --git a/src/lib/isISO8601.js b/src/lib/isISO8601.js
index 1f797347d..d3eda7e13 100644
--- a/src/lib/isISO8601.js
+++ b/src/lib/isISO8601.js
@@ -2,9 +2,11 @@ import assertString from './util/assertString';
 
 /* eslint-disable max-len */
 // from http://goo.gl/0ejHHW
-const iso8601 = /^([\+-]?\d{4}(?!\d{2}\b))((-?)((0[1-9]|1[0-2])(\3([12]\d|0[1-9]|3[01]))?|W([0-4]\d|5[0-3])(-?[1-7])?|(00[1-9]|0[1-9]\d|[12]\d{2}|3([0-5]\d|6[1-6])))([T\s]((([01]\d|2[0-3])((:?)[0-5]\d)?|24:?00)([\.,]\d+(?!:))?)?(\17[0-5]\d([\.,]\d+)?)?([zZ]|([\+-])([01]\d|2[0-3]):?([0-5]\d)?)?)?)?$/;
+const iso8601 =
+    /^([\+-]?\d{4}(?!\d{2}\b))((-?)((0[1-9]|1[0-2])(\3([12]\d|0[1-9]|3[01]))?|W([0-4]\d|5[0-3])(-?[1-7])?|(00[1-9]|0[1-9]\d|[12]\d{2}|3([0-5]\d|6[1-6])))([T\s]((([01]\d|2[0-3])((:?)[0-5]\d)?|24:?00)([\.,]\d+(?!:))?)?(\17[0-5]\d([\.,]\d+)?)?([zZ]|([\+-])([01]\d|2[0-3]):?([0-5]\d)?)?)?)?$/;
 // same as above, except with a strict 'T' separator between date and time
-const iso8601StrictSeparator = /^([\+-]?\d{4}(?!\d{2}\b))((-?)((0[1-9]|1[0-2])(\3([12]\d|0[1-9]|3[01]))?|W([0-4]\d|5[0-3])(-?[1-7])?|(00[1-9]|0[1-9]\d|[12]\d{2}|3([0-5]\d|6[1-6])))([T]((([01]\d|2[0-3])((:?)[0-5]\d)?|24:?00)([\.,]\d+(?!:))?)?(\17[0-5]\d([\.,]\d+)?)?([zZ]|([\+-])([01]\d|2[0-3]):?([0-5]\d)?)?)?)?$/;
+const iso8601StrictSeparator =
+    /^([\+-]?\d{4}(?!\d{2}\b))((-?)((0[1-9]|1[0-2])(\3([12]\d|0[1-9]|3[01]))?|W([0-4]\d|5[0-3])(-?[1-7])?|(00[1-9]|0[1-9]\d|[12]\d{2}|3([0-5]\d|6[1-6])))([T]((([01]\d|2[0-3])((:?)[0-5]\d)?|24:?00)([\.,]\d+(?!:))?)?(\17[0-5]\d([\.,]\d+)?)?([zZ]|([\+-])([01]\d|2[0-3]):?([0-5]\d)?)?)?)?$/;
 /* eslint-enable max-len */
 const isValidDate = (str) => {
   // str must have passed the ISO8601 check
@@ -16,29 +18,33 @@ const isValidDate = (str) => {
     const oYear = Number(ordinalMatch[1]);
     const oDay = Number(ordinalMatch[2]);
     // if is leap year
-    if ((oYear % 4 === 0 && oYear % 100 !== 0) || oYear % 400 === 0) return oDay <= 366;
+    if ((oYear % 4 === 0 && oYear % 100 !== 0) || oYear % 400 === 0) { return oDay <= 366; }
     return oDay <= 365;
   }
   const match = str.match(/(\d{4})-?(\d{0,2})-?(\d*)/).map(Number);
   const year = match[1];
   const month = match[2];
   const day = match[3];
-  const monthString = month ? `0${month}`.slice(-2) : month;
-  const dayString = day ? `0${day}`.slice(-2) : day;
+  const monthString = month ? `${month}`.padStart(2, '0') : month;
+  const dayString = day ? `${day}`.padStart(2, '0') : day;
+  const yearString = year < 100 ? `${year}`.padStart(4, '0') : year;
 
   // create a date object and compare
-  const d = new Date(`${year}-${monthString || '01'}-${dayString || '01'}`);
+  const d = new Date(`${yearString}-${monthString || '01'}-${dayString || '01'}`);
   if (month && day) {
-    return d.getUTCFullYear() === year
-      && (d.getUTCMonth() + 1) === month
-      && d.getUTCDate() === day;
+    return (
+      d.getUTCFullYear() === year &&
+            d.getUTCMonth() + 1 === month &&
+            d.getUTCDate() === day
+    );
   }
   return true;
 };
-
 export default function isISO8601(str, options = {}) {
   assertString(str);
-  const check = options.strictSeparator ? iso8601StrictSeparator.test(str) : iso8601.test(str);
+  const check = options.strictSeparator
+    ? iso8601StrictSeparator.test(str)
+    : iso8601.test(str);
   if (check && options.strict) return isValidDate(str);
   return check;
 }
diff --git a/test/validators.test.js b/test/validators.test.js
index d0908ad12..6db0690f3 100644
--- a/test/validators.test.js
+++ b/test/validators.test.js
@@ -11924,6 +11924,9 @@ describe('Validators', () => {
     '2009-10-10',
     '2020-366',
     '2000-366',
+    '0080-05-19',
+    '0008-W51-1',
+    '0009-05-19T14:39Z',
   ];
 
   const invalidISO8601 = [
@@ -11953,6 +11956,9 @@ describe('Validators', () => {
     '2010-13-1',
     'nonsense2021-01-01T00:00:00Z',
     '2021-01-01T00:00:00Znonsense',
+    '0010-02-18T16:23.33.600',
+    '0200-02-18T16,25:23:48,444',
+    'nonsense0021-01-01T00:00:00Z',
   ];
 
   it('should validate ISO 8601 dates', () => {
@@ -11987,6 +11993,8 @@ describe('Validators', () => {
         '2009-222',
         '2020-366',
         '2400-366',
+        '0050-02-15',
+        '0010-05-19T14:39Z',
       ],
       invalid: [
         '2010-02-30',
@@ -12038,6 +12046,10 @@ describe('Validators', () => {
         '2009-10-10',
         '2020-366',
         '2000-366',
+        '0010-02-18T16:23.4',
+        '0010-02-18T16:23,25',
+        '0200-02-18T16:23.33+0600',
+        '0200-02-18T16.23334444',
       ],
       invalid: [
         '200905',
@@ -12072,6 +12084,9 @@ describe('Validators', () => {
         '2009-05-19 14:39:22-06:00',
         '2009-05-19 14:39:22+0600',
         '2009-05-19 14:39:22-01',
+        '0009-05-19 14.5.44',
+        '0010-02-18T16:23.33.600',
+        '0010-02-18T16,25:23:48,444',
       ],
     });
   });
@@ -12088,6 +12103,8 @@ describe('Validators', () => {
         '2009-222',
         '2020-366',
         '2400-366',
+        '0020-366',
+        '0400-366',
       ],
       invalid: [
         '2010-02-30',
@@ -12100,6 +12117,8 @@ describe('Validators', () => {
         '2009-05-19 14:39:22-06:00',
         '2009-05-19 14:39:22+0600',
         '2009-05-19 14:39:22-01',
+        '0009-05-19 14:39:22+0600',
+        '0009-05-19 14:39:22-01',
       ],
     });
   });