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There is no reason someone couldn't change commit dates and be prolific far earlier than December

Lack of pre-december github activity could be an indicator, i.e. no pull requests, no issues, no stars or forks, perhaps even no feed updates, etc.

Lack of github activity may also be an indicator for having a life outside programming. Or using private repos.

Or just being as prolific on Gitlab, Bitbucket or elsewhere. We have to stop taking Github contributions so seriously.