The isClkOn() command checks if the calculator's clock (which is used by most time and date commands) is running or stopped. The result of isClkOn() is a truth value — true if the clock is on, and false if the clock is off — which makes it perfect for a condition in commands such as If:
:If isClkOn() Then : Disp "Clock is running." :Else : Disp "Clock is stopped." :EndIf
The isClkOn() command, though useful, isn't often called for. For instance, there's no need to check if the clock is on if you're planning to turn it on anyway:
:If not isClkOn() : ClockOn should just be :ClockOn
One use for isClkOn() is in functions, which aren't allowed to change the global status of the calculator with commands like ClockOn or ClockOff. Instead, you might do the next best thing, and return an error message if the clock is turned off.