The Year 2000 problem highlights the difficulty of testing whether mainframe applications will function correctly at some future date. Creating a separate LPAR or IPLing a test machine is just not practical given the volume of such tests involving different programmers, applications and dates. While the concept is simple, the development of such a product is difficult. Users may also specify an offset to the real time, which is useful for simulating different time zones on one machine.
Problems can also occur when testing programs under interactive debuggers.
The debugger product itself requires the use of the system clock. If a simulated date is passed to the debugger product, it may terminate with a message that your product license has expired. STCKs are low-level commands which cannot be intercepted and therefore pose more of a problem. Load module integrity is maintained and version management is simplified. However, any assembler programmer could issue a STCK instruction.