If i dont send it back on the 20th then how can i find it on the 15th? Therefore, if i dont find the painting, i dont have the painting on the 15th.
Oh wait, i just realised that i still would have painted it, maybe. Quite right, i would have it on the 21st unless i keep sending it back.
Or...
Although if i find the painting on the 15th but when it comes to the 20th i dont send it back, then there would be no painting to be found on the 15th and i wouldnt have it on the 20th. But i obviously do have it on the 20th if i dont send it back because the fact is that i did find it on the 15th and didnt send it back when it got to the 20th.