I did about the first half of this very quickly, with the N & E filled in easily, but then suddenly slowed down and found the SW very difficult. Like others who have commented, for several clues I could see what the solutions were, but it took me a long time to understand why, but I always eventually got there. 27 was one of those, but when I finally saw the light it became my COD. Unlike some others here, I had no difficulty with "time", but maybe that's because I'm old enough to remember when people were said to have "done their time" after a 27a. Like others, I had never heard of 19d, but the solution seemed obvious once I had the crossing letters, and Google confirmed that the word exists and has the right meaning, so it doesn't feel like a particularly difficult clue to me. On the other hand it didn't feel like a particularly satisfying clue either, nor did several others, so that this will not go down in my annals of first rate crosswords. FOI 20a, which was almost a write-in. LOI 20, though 18d was the last one that I fully parsed.