Where Am I Doing this wrong? TSQL IF Clause

If we look at the requirement of the requestor . It has been clearly specified as,

DECLARE @SellStartDate datetime = '2002-06-01 00:00:00.000'

DECLARE @SellEndDate datetime = '2006-06-30 00:00:00.000'

Refer the post. As per the need i am providing the solution to it and the requestor is accepting the solution.

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Regards, RSingh