I need time to prepare for it and application deadline is near so can I take the test later and submit it or do I have to take it now?
Also does Harvard accepts Duolingo test for undergrad international applicant?
Copied from Harvard website:
"A strong knowledge of English is essential for successful study at Harvard, including the ability to understand and express thoughts quickly and clearly. Although you are not required to take an English proficiency exam (such as the TOEFL, IELTS, etc.), you may submit scores if you wish to do so."
So my conclusion is that if your application shows strong evidence that you are fluent in English and have great expository writing ability and all your essays and supplementals are written well, you probably do not have to worry about this additional piece of data.
However, if you know you are not the strongest writer, the best reader and have trouble finding the write words to express your thoughts, you probably should take either the IELTS or TOEFL test and submit the test score if it is a good one. I wouldn't recommend to anyone to submit a low score. With regards to Duolingo, you should write to Harvard admissions if you are banking on taking that instead of the IELTS. There is no mention of Duolingo on their undergraduate admissions site.
Good luck.
On regards to Harvard, they don´t require any test of English proficiency for International applicants, though they´´d be satisfied with you sending any test scores whether IELTS, TOEFL IBT or TOEFL from Home , Duolingo, or others as stated explicitly in their admissions page. Good luck and may God bless you.
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As for the IELTS submission deadline, I think it depends on your university.ies, hence you should try contacting them directly. Though from my personal experience I´´d recommend you to take the TOEFL instead since it, as well as the Duolingo Test, is far easier than the IELTS and could really be taken with less than a week of study if you have some experience in Reading and Writting, thus you´´d basically only need to study for the Speaking Section which is relatively the harder one.