Employment forms, school forms, travel letters and referral letters are written after an appointment, by the doctor who examined you, and they say what that doctor found.
A certificate is not a product you can buy at reception and we will not write one for a day you were not seen. If you were unwell last week and did not come in, the doctor can write what you tell them and will make clear in the letter that it is what you reported.
Most forms are ready the same day. Anything that needs a result waits for the result.