how to build the buffer memory

I am running a stepper motor in which I need a |counter / memory| that can store the previous value.

For example: The initial point is "0"(which will always remain the same) and set point is "10"(which the user can vary as per their need), so the stepper motor will rotate for 10 sec. After that if the set point value has been changed to "25" then the stepper motor will rotate for 15sec more.

The system will be working in such a way that the last entered set point will be the initial point for the next step.