The Walking Dead will kick off its ninth season next month, but it won't mark the ninth year of the zombie apocalypse. The show hasn't followed Rick and Co. in real time, and it was never entirely clear just how many days and months and years had passed. Fortunately, The Walking Dead finally gave a precise time frame for the zombie apocalypse, and not as much time has passed as you may have guessed.
Now, those photos are taken from before the zombie apocalypse kicked off in the very first episode of The Walking Dead way back in 2010 and the Season 8 episode that saw Rick emerge from a shipping container in the junkyard, so the 632 days doesn't apply to where the show completed its Season 8 finale. Taking into account that Rick was was in his coma for four to five weeks, Rick probably wasn't up and at 'em in the zombie apocalypse for more than 600 days by the time he walked out of that shipping container. Throw in the passage of time between the shipping container and the showdown with Negan in the Season 8 finale, and Rick still probably wasn't conscious in the zombie apocalypse for 632 days.
If you've done the mental math (or used a calculator) for the 632 days, you'll note that less than two years have passed since Rick was chatting with Shane in his squad car before getting shot and stuck in a coma. While obviously the zombie apocalypse hasn't lasted as long in-universe as fans have been watching from home, less than two years still seems awfully short in my book, and not just because Carl managed to go through puberty between the ages of 12 and 14. Rick and Co. have done a lot of traveling throughout the series; accomplishing all that they have in less than two years is remarkable.
If we want to do some more math, it's probably safe to say that about a year had passed by the end of Season 3. Lori would have been pregnant for almost nine months before she died, and Judith wasn't an infant anymore by the end of the season. If that math is correct, then Seasons 4-8 took place in under a year. Those survivors sure accomplished a lot in not all that much time! No wonder it seemed like Maggie was pregnant forever without even beginning to show.
Interestingly, the reveal (courtesy of the show's official Twitter account) that The Walking Dead hadn't even spanned two full years in the first eight seasons makes the upcoming time jump all the more significant. Back in August, one actor revealed that the time jump between Season 8 and Season 9 will be 18 months. The survivors will have spent just about as much time off-screen as they did on screen in the first eight seasons! No wonder they will have made so much progress and advanced their settlements by the time the action picks up again with the super-sized Season 9 premiere.