Set Min and Max value for a time field

[surfbud]

How can I set the minimum value or maximum value of a time field? Precisely, I would like to set the minmum value to 7:00
When i use setMinValue(Date) it blocks the combo..
Here is the code
public class TimeFieldX extends com.extjs.gxt.ui.client.widget.form.TimeField{
.....

...
private void personnaliser() {
setLabelSeparator(ConstantesRPC.SEPARATEUR_FIELD);
setFormat(ConstantesClient.TIME_FMT);
setIncrement(INCREMENT);
setFormat(DateTimeFormat.getFormat(ConstantesRPC.FORMAT_HEURE));
// setMinValue(new Date());
}
...
.....
}