It is well known that static methods with immutable objects as parameters are thread safe and mutable objects are not.
I would contest this. Arguments passed to a method are stored on a stack, which is a per-thread idiom.
If your parameter is a mutable object such as a Date then you need to ensure other threads are not modifying it at the same time elsewhere. But that’s a different matter unrelated to the thread-safety of your method.
The method you posted is thread-safe. It maintains no state and operates only on its arguments.
I would strongly recommend you read Java Concurrency in Practice, or a similar book dedicated to thread safety in Java. It’s a complex subject that cannot be addressed appropriately through a few StackOverflow answers.