I'm sorry, but I'm lost now. I just had a look at Tips and Notes for Pronouns and for Demonstratives but to be honest I did not feel it helped much with the sentence in question.
If I understand you correctly, you're saying that it should be "Bylo to jejich patnáctá zima", just as in English. But then the suggested solution is "Byla to jejich patnáctá zima?". And in another exercise, the solution is "Byl to sedmnáctý týden." So verb and subject agree in these sentences.
So how am I supposed to understand this. Are "Bylo to" and "BylX to" both correct? Only one of them? If so, which one? I'd highly appreciate some clarification. Thanks.
I am also trying to wrap my head around this one (and others like it), when I expect to to be associated with a neuter verb. So the "in my head" rationale here is that, if I imagine the English sentence as "Was the winter their fifteenth?" byla seems to fit, since it puts zima in the subject role. Does this make sense, or am I way off base with this thinking?