Can you please provide a little more detail to help us understand the use-case? Are you suggesting that you would like the WiFi client user to land on a captive portal page and request the user to enter date of birth in order to obtain WiFi/internet connectivity?
Currenlty on cnMaestro there is no option to allow WiFi access to clients using there date of birth. This can be achieved by integrating 3rd party web server which will be hosting the captive portal requesting client to enter Date of birth details.
As Wright-Fi mentioned, the guest portal in cnMaestro currently only allows a fixed set of fields. The upcoming version 2.0 release of cnMaestro will allow adding customized fields. While these won't let you add customized date widgets, they will allow additional free-text fields, so depending on what your requirements are this may fit the bill.
An update on this custom fields feature: based on customer feedback we have decided to enhance it so that fields of various types such as phone number, email address and so on can be added with validation (as opposed to free-text fields with no validation). Unfortunately this means that this feature will not make it to cnMaestro 2.0 but has instead been moved to release 2.1.