HP made a change in the product effective with v11 so that the Start and End Dates for Releases and Cycles is required, and inserted code (that we can't modify) so that when a Cycle is created it sets the Start and End Dates to match the parent Release Start and End Dates.
I don't think there is any way to circumvent this product change. You need to just enter the dates. You can change them later with the Reschedule option.
