Use Date / Time server validation.
you can use dynamic data in the date / time validation for the minimum and maximum dates.
The example is will use uses form elements named startDate and endDate
Add Date Date Validation for the startDate form field, and use the following code for the Maximum date setting:
<?php echo((isset($_POST["endDate"]))?$_POST["endDate"]:"") ?>
Use Date Validation again for the endDate form element, this time set the Minimum Date to use the start date form element:
<?php echo((isset($_POST["startDate"]))?$_POST["startDate"]:"") ?>