Notification timestamps are all in relative time -- 2s, 2m, 2h, 2d, 2mo, 2y, 3y... While it's useful to see that an event took place "yesterday", knowing that something occurred "3y" ago is the opposite of helpful, since the same timestamp might apply to notifications that arrived 364 days apart.
We can debate what the ideal interval is after which we should switch to absolute time. Mac Mail, for example, goes to dates after "yesterday," while Twitter trips to dates after "24h." We can also discuss the proper form notation should take. One thing worth noting is that as long as dates are within the current year, we can safely drop the year from notation. Whether "within the current year" means within the current calendar year or the current trailing 12-month can, again, be debated when the time comes.