Minor fix in yearString condition

Author: skpanagiotis

src/lib/isISO8601.js

@@ -27,7 +27,7 @@ const isValidDate = (str) => {
   const day = match[3];
   const monthString = month ? `${month}`.padStart(2, '0') : month;
   const dayString = day ? `${day}`.padStart(2, '0') : day;
-  const yearString = year < 100 ? `${year}`.padStart(4, '0') : year;
+  const yearString = year < 1000 ? `${year}`.padStart(4, '0') : year;
 
   // create a date object and compare
   const d = new Date(`${yearString}-${monthString || '01'}-${dayString || '01'}`);