How to determine if Vertica::DateADT is null (in UDx c++ API)

Hi,
In Udx c++ API, DateADT is defined as alias to int64, and represent number of days since Vertica epoch (2001/01/01).
It should be some convention how null is presented in DateADT.
Can you post what is convention for representing null in DateADT.
Thank you
Sergey