Stan Lee is the co-creator of some of the most iconic Marvel superheroes, including Spider-Man and Iron Man. Lee's first published work was as an assistant on "Captain America Comics" No. 3 (May 1941), according to ComicVine. He eventually went on to write and produce more than 500 comics for both Marvel and DC Comics, with a number of them in collaboration with legends Jack Kirby and Steve Ditko.

Lee would go on to become the president and chairman of Marvel Comics, as well as its publisher. He left Marvel in 2010 but remained an executive producer for films based on his comics until his death in 2018.

Stan Lee died at the age of 95 on November 12, 2018.

The Marvel Cinematic Universe also commemorated the legend with a post on Twitter, paying tribute to Lee’s contributions to its movies.

"Today we celebrate what would’ve been Stan Lee’s 99th birthday," @Marvel tweeted in honor of the comic book icon. "From all of us at Marvel, thank you for inspiring generations of fans and creators alike."

Lee would have celebrated his 99th birthday on December 28, 2021.

Fans of the comics series also recognized Lee's contribution to their favorite characters with a number of touching tributes across social media.

"Happy birthday Stan Lee," one fan wrote. "We miss you." Another added: "Thanks for all the heroes that came from your mind. Your work will live forever."

A third wrote, "Stan Lee is why I love comics so much."

But some fans were skeptical of the date on which Lee was supposedly celebrating his birthday - December 28, 1922. "The Twitter account claims today is Stan Lee's birthday but it's actually January 12th," one fan tweeted. "What's up with that?"It is unclear why the Twitter account claimed to be celebrating Lee's birthday on December 28. While it remains possible that the date may have been a typo, many fans were still puzzled by it. "Where's your proof he was born in 1922? How do you know he wasn't born in 1940 or 1941?" another fan tweeted. "Regardless, he's still a legend and will always be loved."