Get the number of seconds and fractions of seconds since the app was started. Accurate to milliseconds, but due to floating point inaccuracies the longer the app is running the less accurate this becomes. More specifically the time value has about 7 significant figures of accuracy, so as the number of seconds gets higher the number of decimal places decreases. If you only need the time to the nearest second then this value will be fine up to about 16,000,000 seconds.
Calling this command will internally make a system call to get the amount of time passed, so it will change every time you call it.