More of a question for mods than OP but how should this be answered? I mean the question makes some assumptions that aren't true, so would you just explain that for as long as there's been soccer, the length of the match could be determined?

I mean timers (for instance the hourglass) predate soccer by millennias. In the 1870s they settle on 90 minute matches, which would have been something relatively easy to time in the 1860s had they agreed from the beginning

As to going to antiquity or cultures that didn't have the means to easily time the game, then the answer is obvious- they played to something that could easily be determined. Whether it's first to 8 like in Ulama, nine innings like baseball, or H-O-R-S-E in horse, not having a clock doesn't stop games from being played