It's definitely subjective, depending on a person's culture and history.
A name can be trendy AND timeless at the same time, which is confusing! Like
Helen, or
Emma, or
Noah, or
Arthur.
For me a timeless name is not necessarily a name that could be from any time period (but sometimes it can be), but one that works on a person at any age. If I see a name and can't guess a person's age or generation right away, it's timeless.