Andy, 

in fact the noun ‘ mi-temps ‘ can be 

la mi-temps, invariable feminine, referring to 'the half ' ( one period of play of the match )  or ‘ half-time' (the mid-match break in play) ‘ -  in sporting events (as used in this exercise)

or 

le mi-temps, invariable masculine, translating as ‘ part-time ‘, in a work setting. 

Yes, the noun ‘ le temps’ is invariable masculine in all its senses.

 https://www.larousse.fr/dictionnaires/francais/mi-temps/51804

 https://www.wordreference.com/fren/temps?start=100

 https://www.larousse.fr/dictionnaires/francais/temps/77238

 https://www.wordreference.com/fren/temps