Doctors know how long you have left to live because of the persons diagnoses and how long other patients have lived with that same diagnoses of the condition.
Certain illnesses and diseases can cause death in patients in a short amount of time such as a year or 5 years or sometimes in weeks.
The doctor takes your diagnoses and your condition and also the time other patients have lived after that diagnoses and determines the amount of time you may have to live with those conditions.
Doctors can't say exactly when you'll die from your illness or condition but they are usually pretty close.
Some people have lived past the time that the doctor has said they had left to live.
So you may live longer than the doctor says.
Doctors typically estimate a patient's likelihood of being cured, their extent of functional recovery, and their life expectancy by looking at studies of groups of people with the same or similar diagnosis.
Overall, doctors' predictions were correct to within one week in 25% of cases, correct to within two weeks in 43%, and correct to within four weeks in 61%.
How do doctors know a patient is dying?
Pulse and heartbeat are irregular or hard to feel or hear. Body temperature drops.
Skin on their knees, feet, and hands turns a mottled bluish-purple (often in the last 24 hours)
Breathing is interrupted by gasping and slows until it stops entirely.