The ClockOn command turns on the calculator's clock, used by most time and date commands. Only when the clock is on, will the value returned by startTmr(), getTime(), and other commands actually change with the passage of time. If you write a program that uses any of these commands, be sure to include this command at the beginning of the program.
Since it modifies the global status of the calculator, ClockOn can't be used inside a function. If you're writing a function that depends on measuring time (so you need the clock to be on), the best thing you can do is use the isClkOn() command to check if the clock is on or off, and return an error message if it's off.
450 - Invalid in a function or current expression happens when ClockOn is used in a function.