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:
1 2 3
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:
Now you can use the –contains switch to find the earliest tag that contained this particular commit:
See my previous post on git for more useful git commands.