Radio's date / time macros need to be configurable. For starters, the timestamps in my posts should be configurable to include the timezone and GMT offset. The Internet audience is global, it's natural to expect that many of my readers will be in another timezone and not be aware of which timezone my content was created in.
There are people who want 24-hour times, as discussed here and here.
Months ago, Ingo Rammer pointed out the issues of using Radio to publish an English-language web site from a German version of Windows.
Ideally, Radio would default to using the regional settings as specified by the OS. I don't mean "It's German windows, we'll use German day names." Windows has a "Regional and Language options" control panel that lets the user specify any language and use any format, and there are APIs to handle the dirty work (presumably Mac OS has something similar). Radio should respect those exact settings, and provide an easy way to override them.
TZ offsets are an acknowledgement of the global nature of the Internet. Most Internet RFCs require either offsets or that GMT be used whenever time data is exchanged.
I'd also like to see better support for calendaring.