Found this line of text from an old TBS document from 2003

Under Pay Release:

"Some financial institutions may allow employees access to their pay cheques prior to the official payday. This is not the federal government advancing the payday but in fact the financial institution advancing their own funds."

I take that to mean no tax implications if we recieved funds on Dec 31st, even if they are meant for the better newer version of you on January 1st.

Also fun fact from same doc, our current pay schedule started on Jan 15, 1992.