Datepicker - Years Working Only Showing 20 year periods

Datepicker - Years Working Only Showing 20 year periods

i have an odd problem. I have a date picker set to run from Jan 1st 1922 - Current Date
However instead of getting a scrolling list of years that goes back to 1922 my year selector only goes backwards in multiples of 20 years.

This code is going into a WordPress plugin as part of a front end form - and I am suspecting some kind of conflict but I was wondering if anyone had any ideas

The formatting code is

 jQuery('.calendarpicker').datepicker({
                changeYear: true,
                changeMonth: true,
                minDate:  new Date(1922, 1 - 1, 1)
           
        });