It does.  But not without a way to represent am/pm.  Try:

    string times = string.Format("{0:htt}", startTime);

or even more concise:

    string times = startTime.ToString("htt");

The restriction seems arbitrary to me.  Seems like a user ought to be able  
to produce whatever string they want, even if it results in the loss of  
information.  But apparently the .NET designers felt differently.  What  
really bothers me is the inconsistency.  The format "hh" by itself loses  
the same information, but is allowed.

Anyway, there's a better way to do what you're doing anyway so the  
inconsistent API shouldn't matter.  :)

Pete