I think you can refer to time measurement in the poem without fear.
Time measurement is as old as astronomy. If by Middle Ages you are referring to the 5th century, then surely the stars and the heavenly bodies were studied long, LONG before the Middle Ages. My history is somewhat rusty, but certainly by the 5th century someone would have used a number of things to measure time: hourglasses, sundials (which were used 3000-4000 BC), candles, just to name a few. Hope this helps.
I know this may be a silly place to ask this, but were they able to tell time in the middle ages!? I need to know for a poem I'm making for english class. To be more specific: did they have the means to (practically) measure an hour's worth of time?