Do not use String#padStart()

It might not be available on older NodeJS and browsers. It can't
be added by the Babel runtime because it is an instance function.
Only Babel polyfill can add such functions and we can't use plyfill
because it changes the global scope.

Author: lutovich

src/v1/internal/temporal-util.js

@@ -383,7 +383,13 @@ function formatNumber(num, stringLength = undefined) {
   if (isNegative) {
     num = num.negate();
   }
-  const numString = num.toString();
-  const paddedNumString = stringLength ? numString.padStart(stringLength, '0') : numString;
-  return isNegative ? '-' + paddedNumString : paddedNumString;
+
+  let numString = num.toString();
+  if (stringLength) {
+    // left pad the string with zeroes
+    while (numString.length < stringLength) {
+      numString = '0' + numString;
+    }
+  }
+  return isNegative ? '-' + numString : numString;
 }