Update 16 Feb 09: Cory and Barry point out in the comments that the time dialogue isn't there to change the 'main' time tag, i.e. the one that will control whether an element is visible or not when you use the timeline (the one that's been around in earlier versions of GE) but its to do with the new 'gx' time tags. Read their comments for details but I will say here I think this isn't great for usability, a dialogue labelled 'time/date' is going to cause confusion for people who won't know which time tag they are editing in properties.
I haven't seen this flagged anywhere on the web yet so a quick tip:
Instead of having to edit the KML to add a time tag to your data so it can be viewed with the timeline you can now edit it directly in Google Earth:
- Right click in the places column > Properties > View > Date/Time.
I actually got a friend to build me a little .NET application so I could edit time tags in KML as it was so tedious to do it manually (and I kept on making mistakes). Neat that this has been added in GE5.