Don’t risk not getting your student application proposal submitted to summerofcode.withgoogle.com! The deadline is March 31 18:00 UTC, but a slow Internet connection, illness in the family, or any of hundreds of other excuses won’t be a reason to accept a late proposal submission. No late proposals will be accepted.
Submit now. Your submission as well as your interactions with mentors to this point will be the key deciding factors in getting accepted, though additional interactions during the slot request time is welcome and proposals can be amended. However, those amendments will be for the purpose of clarification and the mentor/organization review process may not consider those amendments.