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I would suggest displaying an error that advices the user what they would do -
If you would like to implement that, please do the following:
1. Add a form calculation widget to your form - I renamned it as 'compare dates':
2. Add the calculation to subtract the difference between the delivery date and the pickup date:
3. Add the message you want displayed when the user selects a Pickup date ealier than the delivery date:
4. Next - add conditional logic rules to show/hide the message as shown below:
The rule states that if "Compare Dates" contains a negative symbol, then show the error message text field.
5. Lastly - make the calculation field hidden so that it does not show on the form:
Here is the demo form so that you can see how it works: http://form.jotformpro.com/form/41212500749951
Please let us know if this helps.