About Linux kernel printk timestamps

What is this timestamp in each kernel printk message?

Quote from [1].

Printk-time uses the routine sched_clock() in the kernel. On some platforms, sched_clock() only has a resolution of 1 jiffy (which may be 10 milliseconds or longer). This means that you will only see the time increment at this resolution, giving imprecise results for printk-times. To correct this problem, the best solution is to implement a good sched_clock() routine for your platform. Sched_clock() returns a 64-bit value which is nanoseconds since some event (usually either since machine power-on, or since time_init() was called.) Many embedded processors have a clock or timer on the System-On-Chip which can provide a good resolution clock source for sched_clock(). It is best if the clock can provide resolution better than 1 microsecond. Note that this only requires a clock running at 1 MHz to achieve this resolution.