Since the computation function is called to build the value whenever the value is required (that's the purpose of computed
, after all), you simply add that within the function:
//extract Primary dates from entities
report.PrimaryDateRangeAttributes = ko.computed(function () {
var rv = $.grep(entity.PrimaryAttributes(), function (item) { return item.DataType() === 'datetime' });
rv.push('DateEntered');
return rv;
});