I was surprised for a second to see that so many people didn't immediately know what was happening, but then I remember not everyone has personal experience with being suicidal lol. that's a good thing!
The reason for that particular date one year later is because she always planned to die that day. It's quite common for suicidal people to plan, even a long time ahead, for their death. Often, it's a satisfying length of time (in exactly a year, a month, on christmas, etc) perhaps because the person themselves are sure they don't want to live anymore, so a time limit is convenient for them.
Whether she herself made the post (it's mentioned that it's a random blog post) or just latched onto it as a convenient moment to end her life, that's unknown. She told her friend (detail: while smiling) about this supposed end of the world, which tracks perfectly with how real life depressed people sometimes hint at their plans as a last ditch cry for help. The friend either willfuly ignored it or just didn't understand. The latter seems more likely, even though ignoring the problem was convenient for her, it's unlikely due to her age that the author wanted us to think it was fully conscious.
By the end though, her friend realises completely what's happening; the fact that she benefitted from it, and how it's too late now. She's unable to stop her from dying, and too scared for a lover's suicide. Her friend is swallowed by the sea, and she's swallowed by guilt.