Manny offers to sell Gina his television for $$100$ on January $1.$ On January $2,$ Gina writes out a letter of acceptance. On January $3,$ Gina drops the letter in a mailbox. On January $4,$ a postal worker gets the letter out of the mailbox and takes it to the post office. On January $5,$ the letter arrives in Manny's mailbox. When $($if ever$)$ was a contract formed?