Not to undermine you, but is it possible you could have change it on accident somewhere? Or maybe even a process or program you installed or ran recently. Better yet, the time server you sync to, could it be off, could it cause your server to change it's year by that much?
I don't think it's an hack attempt this kind of problems sometimes crops up in plesk panels due to some bugs. You can open a trouble ticket with plesk and get the advise from them. Also it's advisable to have your system admin to do a initial security audit in your server asap.
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