Whether a cardinal or ordinal number to use when reffering to a year?

Hello,

Whether a cardinal (one, two, three, ...) or ordinal (first, second, third, ...) number must I use when reffering to a year? Or it depends on a particular sentence? (The Chicago Manual of Style says that if I refer to a specific year, I must use a cardinal number, although it can be pronounced as ordinal. However, I'm not sure that I understood it right.)

Thanks in advance.

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Victor