Help setting up a event registration with FREE t-shirt

I have set-up an event registration form and it is working, however, I need to add an option for a free t-shirt for all registration before a particular date.

I need to know the best way to set this up.  The is only one t-shirt style with multiples sizes and the cost is free.

Can you provide me with some suggestions.

I've created a demo form for you. I hope you like it.=) First of all, I've created different fields for adult and children as you see below:

Then, I've created condition to hide fields based on selection as you see below:

Here is how it works:

You can clone and check your demo form.

https://form.jotform.com/62063876375969 

If you have any question or issues, please do not hesitate contacting us. We will be glad to assist you.