I think it would actually be 2036 because the copyright limit is 70 years starts when the artist who created it dies.
That's only for works published after January 1, 1978 which fall under the Copyright Act of 1976. Anything published December 31, 1977 or earlier falls under the Copyright Act of 1909.
As others have noted above, for works from 1978 and newer the "70 years after last surviving author's death" only applies for individuals who do a project on their own. For works done "for hire" for a company the work is protected for 95 years from when it was published or 120 years from when it was created (whichever is shorter)
However, none of that applies to what we are talking about because anything published earlier than 1978 falls under the Copyright Act of 1909. That Act has no "death of author" provision. Works are protected for a set length of time from when they were published. Originally that Act protected copyrighted works automatically for 28 years from when they were published and then you could file for a single 28 year extension. So your work could only be protected for 56 years. That means the first Mickey Mouse cartoons (published in 1928) would start to lose their copyright protection in 1984 under this law.
The 1970's had arrived and a lot of earlier films were in danger of losing their copyright protection. Step in Disney's (and other Hollywood studios) lobbyists.....
In 1976 (as part of the work for the Copyright Act of 1976), Congress amended the terms for any works falling under the Copyright Act of 1909 to extend protection to 75 years from publication for any work published in 1923 or newer. That 1923 date was chosen because any work published 1922 or earlier would already be in public domain by the time this Act went into effect in 1978 based on the 56 year protection afforded to those works. Congress could not pull a public domain work back into copyright protection.
So with this, Mickey Mouse was safe for now. His protection expiration went from 1984 to 2003.
Years go along and 2003 is coming up quick. Plus works from 1923 would fall out of copyright protection at the end of 1998.
Queue the political arm twisting and campaign fund money transfers again....
In October 1998 Congress passed (and the president signed) the Sony Bono Copyright Term Extension Act which extended protection for works published after 1922 (and before 1978) to a term of 95 years from publication.
So, with this, Mickey's protection went from 2003 to 2023.
And that's where we sit today.... for now. The first two Mickey Mouse cartoons, Steamboat Willie and The Gallopin' Gaucho, will lose their protection at the end of 2023. With that 2023 date looming, and with 1923 works set to lose their copyright at the end of next year, expect the political lobbying efforts to go into full force soon to pop another extension on the expiration date.