Question: In the Brady guide it lists certain times for street crimes but time never seems to move! How does the day/night cycle work in L.A. Noire?
Answer: The time of day in Story Mode is always scripted; the current time is always whatever time it is in that chapter of the story. You can see it in the upper-left of the cut scene before each mission.
When you go back to do street crimes in free roam, time does progress from day to night and back again and all of the street crimes already completed in story mode will be available. Although there is no clock in Free Roam, you can estimate using the light/dark cycle to know when crimes will occur.
If you are using the Brady Guide and know what time a street crime occurs, you can look for a particular street crime event during the story when the preceding cut scene shows that time. Alternatively, you could go into Free Roam for that desk and find all of that desk's crimes at the proper time during the organic day / night cycle, provided you have already passed that point in Story Mode.
If you are trying to complete crimes in Free Roam quickly, one tip is to exit the mode after completing a street crime and then re-enter. The next crime should be available on the map provided you have already encountered that crime in Story Mode. Note that street crimes that have been completed will show up as grey icons on the map while new street crimes show up as red icons.