@Tkr 

I don't think many 'residential' routers have that function?

I know my modem from my ISP can show me that, as can Windows for the PC, but not directly on the router.

Indirectly, one can look at the LOG and see the date/time of the last boot. However, the size of the log generallly is set by the f/w, and depending on what is getting logged, you might need to e-mail yourself the log daily (or more often) and find the first boot time and then daily check the log to see if it re-booted and do the math.

Seems to be a common question here, Google "tp-link router uptime".