Hello,
i found a bug in the lib when you use the UNTIL Value with a year that is smaller than 10. You can comprehend this bug on the demo page if you put the following value in the UNTIL Input:

10.10.0009, 10:10
will generate
RRULE:FREQ=WEEKLY;UNTIL=00,91010T101000Z

And if you want to parse this string with the fromString-function the lib throws the error:

=> Error: Invalid UNTIL value: 00,91010T101000Z

For me this bug is critical, because when you use the date-input and start typing the year, the lib throws this error.