Can't remove a date range from event

Some of my events are currently on my timeline as a date range, for example 29 April to 2 May 2009. I have decided that instead, I want it just to display the start month and year (eg April 09).

I can set the start date to just the month and year, but the end of the range still shows up and I can't get rid of it (I've tried deleting the end date and saving, with no luck). In the example given, I'm now stuck with an event displayed thus: Apr 2009 to 02/05/2009.

Several of my events behave the same way, and I don't really want to delete them entirely and then re-add them to the timeline. Any ideas?