Alice makes an offer to Brian through a letter posted on Tuesday. At the end, the letter says: "I look forward to receiving a yes reply by the end of this Friday. We will have a deal only when I have received the reply from you." Brian decides to accept the offer and posts the letter of acceptance at 4 pm on Wednesday. But Alice changes her mind. She calls Brian on his phone at 5 pm on Wednesday and says "I revoke my offer". Brian's letter of acceptance is delivered to Alice at 9 am on Thursday.

is there a valid contract? if so, when did the contract come into effect?