Currently info.magnolia.jcr.util.PropertyUtil.createValue(String, int, ValueFactory) reimplements String (ISO8601) to Calendar conversion. There's no need to do so. JackRabbit or JoDa have API for such conversion as have many other libraries (e.g. JAXB).
JackRabbit:
Calendar cal = org.apache.jackrabbit.util.ISO8601.parse(s);
Calendar cal = org.apache.jackrabbit.core.util.ISO8601.parse(s);
JoDa:
Calendar cal = org.joda.time.format.ISODateTimeFormat.dateTimeNoMillis().parseDateTime(s);

Current code is ugly and doesn't handle time zones properly (hence the bug and not just improvement).