When timeformat is defined as HHmm, you cannot type more the 2 characters

Steps to reproduce
1. select all characters of default value
2. start typing new time in HHmm format
Expected: You should be able to input time in HHmm format eg. 1212
Actual: You can only type 2 characters not more than that

<zk>

Timebox: <timebox id="tb" cols="14" format="HHmm" onCreate="self.value = new Date()" mold="rounded"/>

<checkbox label="Use rounded timebox" checked="true"

onCheck='tb.mold=self.checked? "rounded": "default"'/>

</zk>