You can use JavaScript and Luxon. Load Luxon in the survey header:
<script src="https://cdn.jsdelivr.net/npm/luxon@3.4/build/global/luxon.min.js"></script>
Add this to a question (change startDate field name and date format as needed):
Qualtrics.SurveyEngine.addOnload(function() {
var DateTime = luxon.DateTime;
var startDate = DateTime.fromFormat("${e://Field/startDate}","MM/dd/yyyy");
var tenure = DateTime.now().diff(startDate, e'months','days']).toObject();
Qualtrics.SurveyEngine.setEmbeddedData('tenure',tenure.months);
});
The field ‘tenure’ will be the number of months.