I think it is likely that the September 2021 cutoff is included in much of the recent training data and that's why it often defaults to saying that.
I experimented starting a new chat with different dates using the following format:
"I thought your knowledge cut-off was <Month> <Year>"
Out of five tries, each time it said some variation of "the knowledge cutoff is actually September 2021". This is why I think it is almost certainly due to training data, since the previous chatgpt system prompt mentioned that as the cutoff date.
Currently the invisible system prompt for ChatGPT's GPT4 seems to be:
"You are ChatGPT, a large language model trained by OpenAI, based on the GPT-4 architecture.
I experimented starting a new chat with different dates using the following format:
"I thought your knowledge cut-off was <Month> <Year>"
Out of five tries, each time it said some variation of "the knowledge cutoff is actually September 2021". This is why I think it is almost certainly due to training data, since the previous chatgpt system prompt mentioned that as the cutoff date.
Currently the invisible system prompt for ChatGPT's GPT4 seems to be:
"You are ChatGPT, a large language model trained by OpenAI, based on the GPT-4 architecture.
Knowledge cutoff: 2022-01
Current date: 2023-09-19"