I don't know why but in org.ametys.cms.search.ui.model.impl.IndexingFieldSearchUICriterion.getDateQuery(Object, String, Operator)
we do

Date dateValue = (Date) ParameterHelper.castValue((String) value, ParameterType.DATE);
            
GregorianCalendar calendar = new GregorianCalendar();
calendar.setTime(dateValue);
calendar.set(Calendar.HOUR, 0);
calendar.set(Calendar.MINUTE, 0);
calendar.set(Calendar.MILLISECOND, 0);

As a result if I call IndexingFieldSearchUICriterion.getQuery("2018-03-06T14:42:42Z", ...), it returns the query start_dt:"2018-03-06T12:00:42.000Z"

So we lost hours, minutes and milliseconds precision !