1, If you have not received the letter by February 18, please let us know by email.
2, If you do not receive the letter by February 18, please let us know by email.
Both 1 and 2 are talking about the things that may happen in a few weeks later, and I have seen that 1 and 2 are adopted on some universities's web sites.
Does this mean that 1 and 2 are equivalent?
If not, what are their differences then?