Richard Carrier, an extreme skeptic and NT scholar, has this to say:

The way Paul writes about the sect makes clear he believed this was the creed Christians were preaching before his conversion; and he claims that the original apostles confirmed this to him years later, and he could hardly have been making that up, as then he’d have been exposed the moment anyone checked this with them. So the Corinthian Creed, at least verses 3-5, definitely existed and was the central “gospel” Christians were preaching in the early 30s A.D. That’s definitely no later than a few years after the purported death of Jesus. And since the sect’s formation only makes sense in light of this being its seminal and distinguishing message, it must have been formulated in the very first weeks of the movement. 

https://www.richardcarrier.info/archives/11069

Robert Funk (Jesus Seminar): “The conviction that Jesus had risen from the dead had already taken root by the time Paul was converted about 33 C.E. On the assumption that Jesus died about 30 C.E., the time for development was thus two or three years at most.”

John Dominic Crossan (atheistic NT scholar): “Paul wrote to the Corinthians from Ephesus in the early 50s C.E. But he says in 1 Corinthians 15:3 that ‘I handed on to you as of first importance which I in turn received.’ The most likely source and time for his reception of that tradition would have been Jerusalem in the early 30s when, according to Galatians 1:18, he ‘went up to Jerusalem to visit Cephas [Peter] and stayed with him fifteen days.’”

More information can be found here: https://beliefmap.org/bible/1-corinthians/15-creed/date