"x days later"裡頭的later一字具有「相對性」的意思，在使用時必須相對應於一個已知的時間點才有意義。比如:
Our dog disappeared last Saturday. Then, he showed up from nowhere two days later!
這短文的開頭首先指明了「星期六」，然後「兩天後」狗出現了，這時就可以用two days later，平常的日常對話也是如此。如果你沒頭沒腦的直接說出什麼什麼"x days later"，那麼聽的人會搞不清楚你在說什麼的幾天後。
若是要說今天算起的「幾天後」，那麼就用"x days from now"或是"x days from today"。那麼「大後天」要怎麼說呢？-- three days from now。這還有其它表示法，比如"in + 一段時間"--I'll see you in four days.(四天後碰面/見面/四天後我會回來等等)
當然，如果已知是「今天」，那麼可以用three days later代表大後天。比如你去看醫生，醫生交代今天先做測試，然後三天後回來複診，這時醫生可以說"Take this test first (today), then come back to see me three days later (to get the results)"。
另外參考這本書的一小段截錄:
"Practical English Usage" by M.S., 3rd edition
http://www.englishforums.com/English/In90DaysWithin90Days/jcprq/post.htm
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However, in normal AmEng usage, "in 90 days" is usually equal to saying "in about 90 days", and is seldom meant to convey the level of precision implied by your August 31 date.
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