Get time from boot to FreeRTOS

I am using http://www.freertos.org/ for the application, but I cannot find how to get the system time since boot. I can create a task and constantly update the counter, but I don't think that would be good as the scheduler might schedule my task (and adding a higher priority might hang my "real" work tasks).

So, I want to know what might be the best solution for getting how many claims have passed since the system boot.