Can pulmonary function test for COPD be completed n given results within a day? How accurate is the test to diagnose copd?

Maybe... The pulmonary function test is performed by a respiratory therapist (usually) and then a pulmonologist interprets the test and sends the result to your doctor. Your doctor may not get the result on the same day as it may take longer than a day for the test interpretation to occur. It is a very accurate test for copd.
Limited use. This is actually a measure of a few lung functions on a particular day. They point to but do not prove particular causes of breathing ills. To use pulmonary function tests to diagnose "COPD" (a term that includes a couple of widely-different lung diseases) is like using a yardstick to guess the cause of short stature -- you need the clinical picture, not just numbers. Results are avialable instantl.
Yes, good. Pfts can be read in minutes, and take 10-30 minutes to perform. If you get them just prior to your visit, bring them and your pulmonologist can read them for you right away. The tests accuracy is good as long as the center regularly tests their machines and calibrates them.