I have a Polar F6. I got it in late 2005, so yours might be a bit different, but I don't see a way to track time in multiple manually-defined zones during a single session. It lets you set a target zone manually, so you can track how much time you spent in that zone during one session. The closest thing I see is that you can go to File, Diary, HR Zones, and it will show you the weekly totals for time spent in each of the three pre-defined zones, but those probably only have meaning if you use the "Automatic" or "Ownzone" options, not manual.
I think what you'll have to do is set the zone manually for whatever range you're targeting and start recording. When you want to change zones, stop the recording, reset the manual zone, start recording again, etc. Each such recording session will be stored as a separate exercise in the "File" section that will give you the time in zone for each of them. It will store the last 12 exercises.
If you're looking to exercise for a specific amount of time in a given zone, the unit will show you the time you've been in the target zone while you're still riding.
If you want to set a bunch of zones manually, start the monitor, stop when you're done, and then read off the time spent in each zone, you'll have to upgrade.