Warning: spoilers for 13 Reasons Why ahead.
When the series begins, high school junior Hannah Baker (Katherine Langford) has killed herself. After her death, tapes that Hannah recorded get passed around the school — tapes in which Hannah details the "13 reasons why" she decided to end her life. Hannah's tapes span the course of roughly a year in her life, from her sophomore year as a new student into the second semester of her junior year. However, there's something interesting about that timeline that eagle-eyed writer Bridget Serdock noticed while viewing the series.
Towards the end of the show, certain students are called to give taped depositions in Hannah's parents' lawsuit against Liberty High School. There's a timestamp on the deposition. Zach's (Ross Butler) reads November 10, 2017 — a date that hasn't actually happened yet. Which means that some of the events on the show likely took place in the future (in our real world timeline) as well.
To be clear: the series isn't implying that the events took place in the future. However, it is a little odd that a show with a premiere date in March of 2017 would choose to set its present-day timeline in the future, rather than during the prior year. Serdock thinks there's a very specific reason for that: to promote a message of hope amongst viewers.
While we don't have an exact timeline for when things happened to Hannah, there are certain "clues" that suggest the time of year. For example: the winter dance likely happened in, well, winter (possibly January, after Christmas break?) and that Jessica's (Alisha Boe) party happened in September, as it was the "first party of junior year." While events like the winter dance already passed in the timeline of the real world, the big traumas have not yet occurred. Bryce (Justin Prentice) did not rape Jessica or Hannah. Sheri (Ajiona Alexus) did not knock down the stop sign, which led to the death of too-good-for-this-world Jeff (Brandon Larracuente). If something had changed before these events, it's possible Hannah would have made a different choice.
So maybe that's the point of the timeline: while we can't save Hannah on the series, perhaps the show is suggesting that we still have time to reach out to people in real life. It's not too late to make a difference.