If you need to extend your student visa, the earliest time the immigration rules allow you to make this application is 3-months before the start date in the new CAS. If you are extending your Student visa for the purpose of remaining on your current course, the new CAS date will always be the day after your current expiration date.
For example, if your expiration date is 30th January, the start date of your new CAS is 31st January and it is from this date you will need to backdate 3-months (31st October) for when you would be able to make the visa extension application.
The Student Visa Compliance team will only provide you the CAS for your visa extension 3-months prior to the start date in the new CAS too.