eclipseyesmile's manga / #Happy Ending(1)

I was just wondering, is the day they bury the capsule supposed to happen before the fated day? If not then why the matured friends from 10 years in the future could read Kakeru letter in the capsule when they dig it up? Is there anything like that "capsule-bury" day mentioned in letters to the past? Hmm... the only thing I can't comprehend.