After talking with British and then posting a follow up to the community, we have resolved the issue.

Goal: We have a variable set shared across multiple catalog items. We needed to set two date fields in this variable set to a date X number of days out based on a schedule. The number of days out is determined by the "Delivery time" on the catalog item.

Issue: Scripts were conflicting with OOB ServiceNow date formats end users can select.

Solution: Set the dates using the "Default value" field on the variable definition, putting javascript in this field. This is the script:

javascript:

// Get the datetime now

var nowGdt = new GlideDate();

// The name of the schedule

var myScheduleName = '9-5 weekdays excluding holidays';

// The basis of our calculation

var dueDays = 0;

var cat = new GlideRecord('sc_cat_item');

cat.addQuery('sys_id',gs.action.getGlideURI().getMap().get('sysparm_id'));

cat.query();

if(cat.next()){

var dur = cat.delivery_time.dateNumericValue();

dueDays = dur/24/60/60/1000;

}

var dueWorkingHours = 8;

// The amount of time we want for the duration

var dueSeconds = dueDays*dueWorkingHours*60*60;

var leadTime = new GlideDuration(dueSeconds*1000);

// Calculate the Due Date!

var dueDateGdt;

var schedRec = new GlideRecord('cmn_schedule');

if (schedRec.get('name', myScheduleName)) {          

        var sched = new GlideSchedule(schedRec.sys_id);

        dueDateGdt = sched.add(nowGdt, leadTime, '');

}

dueDateGdt;