How does github determine the first commit date of a file?

Question

I'm not very familiar with github and it's web interface except how to do basic operations (view the uploaded files, download as zip, fork project)

Recently Microsoft open sourced the GWBASIC assembly source code, and all the files in the project show that they were committed 38 years ago except README.md which was added 5 days ago

I always thought the commit date was based on when the file as first uploaded to github, but doesn't seem to be the case. What is happening here, or did github actually exist in 1983?

Microsoft announcement

Answer 1

GitHub didn't exist in 1983; but it's worse: Git didn't even exist until 2005. What happens here (or in similar cases) is

• an import/migration script from another source control system (that doesn't seem likely here, given that there's only one commit)
• one of many tricks to change the date of a commit

This commit date in the Git repository determines exactly what is shown on the GitHub page.