When I was adjusting FP times for next month's trip, I found that in one instance I could make the FP for a time about 15 minutes away from what I wanted- it was the closest time available. Then I went back to modify, and a time 5 minutes closer to the time I wanted was available, so I changed it to that. Then I hit modify again, and by doing this repeatedly in 5 minute increments, was able to get to the exact time I originally wanted (which was not available when I first looked). Worked for me a few days ago, so it's worth a try.