We all have an 'internal clock' which is quite accurate when it's under an hour. I remember a study in which people were placed in a waiting room with no clock for different amounts of time and then asked how long they waited. They were quite accurate when it came to the differences between 5, 10, 20, 30, 45, 60 minutes, but after that it gets hard to tell. So if you read the time on your clock and then have a WILD and see the time, it is not surprising it would be accurate sometimes in the dream, and we can make a very good guess at how long the dream lasted using our 'internal clock'.
For fun, after a lucid dream when I WBTB and see the time before I went to sleep, I wake up and guess what time it is before I verify it on my clock, trying to judge how long I was asleep for. Sometimes I'm close and other times I'm surprisingly way off!
Anyway, back to clocks in lucid dreams. Ever notice how we use the word, "read" time? Perhaps the reason why they are askew in dreams is for the same reason why written text is hard to read. Perhaps they both use a certain part of our mind that is not working right. (Of course full clarity and awareness in a lucid dream will open up that part of the mind and correct the problem). But most of the time, clocks in dreams are all gobbledygook to me too!
( To use the proper terminology!