I have an event on 12/31/2015 10:00 AM and ends at  12/31/2015 11:00 AM.

But I can compare the datetime with Greater than or equal to
select Id, Subject, isAllDayEvent, StartDateTime, EndDateTime,venue__c from Event where  startdatetime >= 2015-12-29T01:30:00.000+0000,

But i want to do the same with  Less than or equal to or even Less than it's not working.

Please, someone, guide me on this to avoid de-duplication of event,
should give error msg when user, is trying to create a event  in between of startdatetime and enddatetime of a created event.