There are time limits ('caps') on how long you can study with a Tier 4 visa. Before they issue your CAS, a Tier 4 sponsor will check that your proposed studies will not take you over the cap. When assessing your application, the Home Office will also check that you will not exceed the cap, unless there are "compelling and compassionate circumstances", such as injury or disability.
Both your Tier 4 sponsor and the Home Office use the same system to calculate the cap in years and months. Individual days or weeks are rounded up or down to the nearest month. For example, three months and ten days is rounded down to three months while three months and three weeks is rounded up to four months.
The Immigration Rules make it clear that the whole duration of your leave (your visa) is counted, including the extra periods of time added to your visa before and after your course. There are no deductions: vacations, time spent outside the UK, any time spent not studying, and any time spent on a different level course are all included.
The Home Office's Tier 4 policy guidance confirms that if your visa is curtailed only the period before the curtailment date is counted.
If you are a Syrian national who is normally resident in Syria and currently living in the UK these time limits will not apply to you, according to the concession for Syrian nationals
. If you need to rely on this concession we suggest that you speak to an immigration adviser before you apply and that you include a letter with your application explaining your situation and asking for the concession to be applied.