"A year and a century"
Translation:Rok i wiek
[What follows is based on some Internet searches and I have no really good sources to cite, but parts are taken from http://sjp.pwn.pl/poradnia/szukaj/rok-lata.html. This might not be very accurate, but I hope it sheds some light on the topic. Comments or more precise answers are welcome!]
It seems, that once upon a time "rok" (plural "roki") meant "the time by which a trial in court is postponed" (note that "wyrok" = "sentence, the penalty imposed by a law court"). "Lato" meant "the warm time of a year" (including a part of spring, so it was longer than the today's season "lato"="summer"). The plural "lata" was used to describe "years" (and while today the most common way to say "He is 13 years old" is "On ma 13 lat", you could say e.g. "On liczy sobie 13 wiosen", i.e. use the name of another season (here: "wiosna" = "spring" (genitive plural: wiosen))).
And then, when using a calendar became more important (late middle ages?) it somehow happend that "rok" started meaning "a year, 12 months" (because it was the time between two consecutive session of the court?), "lato" continued to be a name of a season, and "lata" continued to be used in order to express how old someone is (and became the plural of "rok" = "a year").
PS: another example of a drastic change in plural: "człowiek - ludzie" (but this one is the same in English "a human - people").