There no sense of time in CPU it can not execute something in very specific precise moment of time, instead CPU can only check time by checking it at loop, and UE4 specifically do this on each frame. So if you set timer to 0.001 which would be speed of 1000FPS, it will practically execute on every frame or even start to bug out, so the maximum precision of timer is same as ms counter that you see when you type in “stat fps” in console. Even if you runs some other timer on other thread, there no guaranty it will give you precision of 0.001 as there no guaranty that OS will run your thread in right time on CPU
But UE4 provides delta time on tick (and many per frame events) which is estimated time passed between the frames, it is most precise timer you can access in blueprint and if you add up delta time on tick you effectively will count time. IT allows to do most pricise time calculations in blueprint, like stat smoothly changing per frame. checking timer estimate should be give you same result too.