Hi Felix,
If you are an admin, you should not need the custom datepicker start date extension.
Just a tip: when selecting the year from the dropdown, it will show 10 years ahead and 10 years behind. You can select the oldest date from the dropdown, and clicking it again will show an additional 10 years behind. If you’re going more than 20 or so years in the past, it might be easier to just type the year into the date field manually.
Let me know if this helps!
Thanks,
Sky