It does that all the time whilst the page is still loading. I use the site a lot on mobile as i vote in the mornings and this happens 9/10 lol
How it works is basically, the date is always just there, but hidden. Magic javascript is then used to do clever things to the date, so it shows how long ago it was, rather than the big long boring date. If you're connection is slow, or Aikar broke something the javascript doesn't load correctly for some reason, then you see the big long boring date.