This is my test code: (Tested on zk sandbox as well)

<zk>
<zscript>

Date d = new Date();

public void setValueForDatebox() {
    d = null;
    tm.setValue(d);
}
</zscript>

<button label="Date to null" onClick="setValueForDatebox()"/>

<timebox id="tm" value="${d}"/>

</zk>

Test scenario 1: (Working ok)
Click on button "Date to null" -> timebox goes blank. Perfect!

Test scenario 2:
Refresh the page
Click on the timebox and modify the time on the screen.
Click on the button, setting value to null -> timebox shows 00:00, it should go blank!

Best Regards.