What Columns to create Indexes on?
From the limited information given, a suitable non-unique clustered index is:

CREATE CLUSTERED INDEX CX_dbo_myTbl_StartDate_EndDate_Username
ON dbo.myTbl (StartDate, EndDate, UserName);


That said, there are all sorts of other considerations that might mean a different choice would be optimal for the workload as a whole. If you improve your question with additional detail, it will be possible to give a better answer.

Also, in general, a PRIMARY KEY (clustered or not) is highly desirable. It is a fundamental principle of relational design that each row (tuple) of a table should be capable of being uniquely identified.