You were thinking that what is it, exactly what title is trying to say ?
If so, I also was amazed when I see Aadhaar card number as a Whatsapp contact on my colleague (Priya) phone.
Trick is Simple
Just save your Aadhaar card number on your phone book then open whatsapp application and refresh the contact list and then you will be able to see Aadhaar card number as your whatsapp contact.
Even you can send a message to your Aadhaar card number but unfortunately it won't be delivered (Just one tick) :P .
However, let's come to the point - You can know your issue date of Aadhaar card number.
As you can see on the screenshot, indicate by red box, pointed by arrow.