To explain this situation two Sections of The Indian Contract Act, have to read simultaneously i.e., Section 4 and 5. Section 4 talks about WHEN THE COMMUNICATION OF ACCEPTANCE AND REVOCATION IS COMPLETE. Firstly, the communication of Acceptance as against the Proposer of the contract is complete when The Letter of acceptance is despatched by the acceptor. The same letter will form completion on the part of the acceptor when it is received by the proposer. Thus, coming to the knowledge of the Proposer is a vital point for this question. Secondly, the communication of revocation is complete is as against the person who makes it, when it is put into a course of transmission to the person to whom it is made, to be out of the power of the person who makes it; as against the person to whom it is made, when it comes to his knowledge. Again, coming to the knowledge of the other person is important.
Putting in these two sections to the question, it can be concluded that even if the letter of acceptance and revocation was put on different dates but received by the proposer on the same day. The essential element would the which letter he opens first. By opening one, the effect of the other would be nullified. Thus, to make revocation of acceptance in effect, it should be made to the knowledge of the offeror before the letter of acceptance is opened.