Currently DateComponents and TimeComponents have static DecimalFormat fields that are shared between threads to implement toString(). According to the doc for DecimalFormat

"Decimal formats are generally not synchronized. It is recommended to create separate format instances for each thread. If multiple threads access a format concurrently, it must be synchronized externally."

I haven't seen any practical impacts and the actual implementation may be thread safe in particular JVMs, but for safety we should use these classes inline with their documentation.