You successfully passed a job interview. You are expected to start on November 15, but you will not be available on that date. Write a letter to your new boss,
Explaining your situation,
expressing your concern and
suggesting a solution.
My name is Leo Gomes and I have recently passed a job interview with your company for the Software Engineer position. I was expected to start on November 15, as agreed, and am writing because, unfortunately, I will not be able to present myself on that date.
During the interview I estimated fifteen days as the amount of time needed for me to finish my activities at my current job and start at your company, but due to the fact that I got sick for the last whole week and was not able to work, I could not finish my tasks and will need another week to do so. If necessary, I can provide you a copy of the letter from my doctor informing that I had to stay at home and under medical care during that week.
I feel responsible for the pressure my delayed start may have created on the company, and I am going to work harder until everything is back to normal. To clear the backlog created by my late start, I can work two additional hours every day until I catch up, either coming in early or leaving late.
I sincerely apologize for the inconvenience and hope this will not affect our relationship. If I am expected to present any other form of proof or talk to someone else in the company, please let me know.