fromString() throws an Invalid date error

[sajinshrestha]

Steps to reproduce:

• create a rule using the dtstart date 01/01/0990
• run rule.toString()
• convert the string output back to rrule using RRule.fromString()

Validation fails in untilStringToDate.

const new_rule = new RRule({ 
  dtstart: new Date(990, 0, 1)
});

const rule_string = new_rule.toString(); // this works
// toString() => "DTSTART=9900101T060000Z"

const rule = RRule.fromString(rule_string); // errors
// fromString() => Error: Invalid UNTIL value: 9900101T060000Z

See runkit example .

[sajinshrestha]

new_rule.toString() string is stripping out the leading 0 from the output. It is returning 9900101T060000Z instead of 09900101T060000Z

toString() seems to be doing the right thing.
new Date(new Date(990, 0, 1).getUTCFullYear() will return 990