If you change what you have now to 2017, you're suggesting that none of the content was created earlier.
Of course, none of this matters at all because all of your content is copyrighted from the moment you created it whether you have that line in your footer or not.
I mean, if you really want to include a year (I wouldn't), go with "©️ [Year first content was added]–[Current year]".
ex: ©️ 2014–2017
If you change what you have now to 2017, you're suggesting that none of the content was created earlier.
Of course, none of this matters at all because all of your content is copyrighted from the moment you created it whether you have that line in your footer or not.