By default, classes cannot start any later than the start of a grading period because the nightly scripts check and see if a class's start date does not line up with a grading period and adjusts it automatically. Nightly scripts update the begin and end date fields if the field is blank or if the date in either of the fields is set to within 7 days of the grading period begin/end date. This only occurs for classes in the Active School Year.
You have the option of disabling this portion of the nightly scripts, but it is important to consider that this will disable this data grooming for ALL classes, so you should be extra careful when setting class start dates in the future.
To request that the portion of the nightly scripts that governs this is disabled, please open a ticket with Veracross Support.