Determine deadline for periodic task that enter ready queue in RTOS
You can't generate a deadline from nothing. That doesn't make sense.

Some tasks must be completed by a certain time, or they are useless.  Only the programmer who writes the task knows whether this is the case, and if so, knows when they have to be completed by.  That's not something that the OS can infer, because it depends on what the task is doing and why.

For instance, a task that adjust the jet engine's power level in response to pilot commands might need to respond rapidly to make sure that safety requirements are met.  However, a task that displays frames of a movie on the passenger's entertainment system is probably less critical; if the next frame of video doesn't get displayed or is delayed, there's no great harm done.  The OS cannot distinguish those two cases.  Only the software developer can.

Therefore, in a real-time OS that tries to meet task deadlines, the deadlines come from the software developer.

For periodic tasks, we might not have a fixed deadline, but we might have requirements on how often they run (e.g., must occur at least two times per second).