JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
We believe that if one user has a question, there could be more users who may have the same question. This is why many of our support forum threads are public and available to be searched and viewed. If you’d like help immediately, feel free to search for a similar question, or submit your question or concern.
You won't need a Formula to achieve this, just setup the Properties of the DateTime field as follows:
1. Disable Future date selection:
2. Set the START & END dates as January 1, 1999 (or any date you prefer from 1999) to today:
Here's a short clip on how it was setup and how it works:
And here's my test form so you can see it in action: https://form.jotform.com/62807921838970