I'm going to have a go at answering your question. Please tell us if your question was different.

You can change the hour, minutes of all the Timestamp type attributes in a work item (bar those in Approvals tab -- that object can't be broken down) by defining an attribute customization for a Calculated Value of provider type Script-based Calculated Value. You do that by following the below link and writing a little snippet of Javascript.

Calculated Values
https://jazz.net/wiki/bin/view/Main/AttributeCustomization#Script_based_calculated_values

For example, you could write something like the below to remove the time portion whenever saving a work item:

dojo.provide("com.example.common.RemoveTimeCalc");

dojo.require("com.ibm.team.workitem.api.common.WorkItemAttributes");
dojo.require("dojo.date");
dojo.require("dojo.date.stamp");

(function() {
var WorkItemAttributes= com.ibm.team.workitem.api.common.WorkItemAttributes;
dojo.declare("com.example.common.RemoveTimeCalc", null, {

    getValue: function(attributeId, workItem, configuration) {

        var currentDate= dojo.date.stamp.fromISOString(workItem.getValue(WorkItemAttributes.DUE_DATE));
        //console.log("The value of currentDate is: " + currentDate);

        if (currentDate != null) {
            currentDate.setHours(0);
            currentDate.setMinutes(0);
            currentDate.setMilliseconds(0);

            var isoDate= dojo.date.stamp.toISOString(currentDate, {milliseconds:true, zulu:true});
            return isoDate;
        } else { return null; }
    }
});
})();

You need to alter the WorkItemAttributes.DUE_DATE bit so that it matches the field you are applying it to. But the rest of the script, except for the name can be reused for each Timestamp field.

<Steps>
1. Go to the project area configuration in Eclipse under Workitems > Attribute Customization and add an entry under Calculated Value.
2. Paste the script as is here into the script section on the right.
3. Go to Workitems > Types and Attributes > select the correct work item type > scroll down to your attribute > Edit > add your new calculated value to the Due Date field
4. Repeat steps 1-3 changing the name of the script each time and altering the attribute name. For built-in attributes you can use WorkItemAttributes.XXXXXXX type name, as per https://jazz.net/wiki/bin/view/Main/AttributeCustomization#Accessing_built_in_attributes_of .