[Leherenn]

Synonymous does not always mean "exactly the same", some times it's "nearly the same". Most synonyms have subtle nuance differences I would say.

In this case, if we are talking about hundred-year spans at once, a shift of 1 year is irrelevant, thus I would say "synonymous" is valid.

I hope this soothes your annoyance.

[ponyfleisch]

I get your point, but would you say "a 99 year period" is synonymous with "a century"?

[Leherenn]

Generally yes, though it would depend on the exact context.

The Spanish monarchy in the 1600s or in the 17th century is the same for me, same if you say "the Spanish monarchy (1600-1699)", unless a specific event happened in 1700 (revolution?).

[drdec]

Both the 1600s and the 17th century are 100 years long.

[ponyfleisch]

Yes, that wasn't my point. I find the term "synonymous" when used for unambiguous numerical values that are off by 1% strange. But i'm not a native english speaker.

Roughly 1% of people born in the 17th century were not born in the 1600's.