How can I allow users to pay 50% the full amount and then complete the payment on a later date?

50% is due as a retainer, with the remaining 50% due 30 days before the event. How can I set this up on my form through Paypal?

As I can understand you need to get your users to pay you 50% on the first submission and then allow them to pay you the other 50% 30 days before your event. 

So far this functionality as such is not available, but I think this can be achieved with a workaround, here is how I think it could be implemented. 

First, you will need to create your form as normal, state what will be the full amount to be paid and specify that you will initially charge only 50% of the full amount, to achieve this you will need to use the Form Calculation widget to get only the 50% of the amount and pass it to the payment field so they are charged for this amount. 

Here's a guide that will help you to perform calculation in your form: How-to-Perform-Form-Calculation-Using-a-Widget

This guide may also help you: How-to-Assign-Calculation-Value

And this one will help you to pass the calculated amount to the payment field: How-to-Pass-a-Calculation-to-a-Payment-Field

I have also made an example about how the first form should be created, here are the steps: 

1. First I have added some sample products in a text box field and assigned calculation values (please, check the guide given above to assign a calculation value). 

2. I have added then a Form Calculation widget that calculates the 50% of the full amount only: 

If you need to show them what the total amount will be then you could simply add a second calculation widget and leave the total result: 

3. Once the calculation is complete, then you will need to pass the amount to be charged to the payment field, please make sure you're  passing the 50% calculated value: 

You have the first form ready to be submitted and charge your customers; however, there are some steps where you will take the info already submitted and pass it to the second form where your user will complete the payment, this will be done passing the data from the first form to the second one through the URL, to get more info about this, please check this guide: Prepopulating-Fields-to-Your-JotForm-via-URL-Parameters

Once you have all the info you will need to include it in an email auto-responder (you can send it as soon as the form is submitted or schedule the email to be sent on a later date, this will be shown below). 

On the second form, you will need to add the fields that you need to show to your submitter, and do not forget to pass in the URL the 50% that needs to be paid as well as  the full amount, if you want to show it though you could re-calculate it in the second form. On my example, I have these fields in my second form: 

Then once the user submits the form he/she gets the link to the second form, once they open this second link they look something similar to this:

This way the user will have pre-filled the info he/she already submitted and simply complete the payment. 

Now, you can send this in an email auto-responder and if you want it could be sent immediately or some days after the first payment has been submitted. 

This guide will help you to add the auto-responder email to your form: Setting-Up-an-Autoresponder-Email

On the auto-responder settings you will be able to select when the email will be sent: 

You could even set up two auto-responders, one that will be sent as soon as the form is submitted and the payment completed and a second one that will be scheduled to be sent the days you specify so this will work as a reminder to your submitter that he/she needs to complete the payment. 

Here're my testing forms: 

First Form: https://www.jotformpro.com/form/70046295921961 

Second Form: https://www.jotformpro.com/form/70046995524967 

This guide will help you to clone them if you want to inspect what I have done there: How-to-Clone-an-Existing-Form-from-a-URL

If you have any question, let us know.