It can be very useful to be able to answer this question accurately.

To do this, add code to your deploy script that tags every production deploy with git. Example:

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  timestamp = Time.now.strftime('%Y-%m-%d/at/%H.%M')
  tag_name = "deploy/#{timestamp}"
  `git tag #{tag_name}; git push origin #{tag_name}`

Now, if the question ever comes up when a particular feature went live, you can determine this down to the minute. Find a commit that was part of this feature. For example, a new header design:

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   git log --grep="header"

Now you can use the –contains switch to find the earliest tag that contained this particular commit:

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   git tag --contains <SHA>

See my previous post on git for more useful git commands.