I don't think there is anything set in stone regarding this, it depends what their own policy says on this. The thing is once it is past the day of you being sick it is no longer a certificate providing proof of your medical condition - a retrospective certificate does not provide a first hand opinion of the doctor based on an examination, it is an assessment based on the symptoms that you describe that you had a day or a week before - which is essentially no better proof than you telling those symptoms to your employer. All it certifies is that you have told your doctor those things.