unable to search Events module by date/time

unable to search Events module by date/time

I'm trying to run a query against my Event and filter based on a start date/time.  I always get nothing returned. 

For example:

https://crm.zoho.com/crm/private/xml/Events/searchRecords?authtoken=<my token>&scope=crmapi&criteria=(Start DateTime:2015-06-18 09:00:00)

returns code 4422.

But if I remove Start DateTime and search based on something else -- ie. Subject, I can see records in that time frame. 
Is this something that I simply cannot do? Or is my formatting of the date wrong?