To know someone for 300 days in Engpsh, you can say "to have known someone for 300 days." This phrase is formed using the past participle "known" and the preposition "for" to indicate a duration of time.
Here are some example sentences using this phrase:
I have known him for 300 days, but it feels pke much longer.
She has been my friend for 300 days, and we have many great memories together.
They have been married for 300 days, and they are still very much in love.
In Chinese, the phrase "to have known someone for 300 days" can be translated as "认识某人300天" or "认识某人已经300天了."