apply a datetime converter to your control and set it to show time only.
Update:
Just read your last remark regarding the changing of values. Maybe you are mistaking the role of a Computedfield control in xpages with the one a computed field has within a Notes Form: in xpages the control is like a "computed for Display" field as it doesn't store values in a doc, it just displays them. In your case you might want to use a combination of a beforePageload event where you write the current date time value to the Notes field, if the datasource isNew(). Your computed field then is showing the value stored before.
Update #2:
In case you just wanted to display the time of day when the doc was first created you of course don't need to calculate and store that value in your documents. Instead just use the computedField mentioned above, then apply a ssjs version of @Created
to it. Finally apply the beforementioned Datetime converter to the control.