This happens due to the way we markup the dates and the way we initialize the JS date handling.
For non-JS users, only the date of the timestamp is shown. Additionally, we only initialize the JS dates when the window is focused. It's quite likely that you hovered over alerts while the window was blurred (though the browsers still trigger hover events).
It's something that I've done a number of times actually. I'm not sure if it's something that we can workaround without changing the concept of only updating dates when focused.