If I make donation to Salvation Army online in December 2017, and they pickup in January 2018, when is the donation year?

If I donate items to Salvation Army by their online form in December 2017, and they pickup in January 2018, what is the donation year? 

According to Salvation Army call center, they assured me that the donation is for the 2017 tax year; because "It is not my fault when they are able to pickup the donation that is waiting for them".

As far as I understand, their receipt will show the online December donation date (which I will receive after they have picked up the items in January).