What term describes something that is extremely old and seems to belong to an earlier period?

The term that best describes something that is extremely old and seems to belong to an earlier period is "archaic." This word specifically refers to items, concepts, or practices that are no longer in common use and evoke a sense of a previous era. "Archaic" captures the essence of antiquity and outdatedness, implying a connection to an earlier time that is distinct from contemporary life.

"Antique," while also relating to old items, often implies value or collector’s worth and is typically used in the context of objects that are over a certain age, usually around 100 years. This makes it more focused on items of historical or monetary significance rather than simply being old or from a past period.

The term "modern" is in stark contrast to what is being described. It indicates contemporary, current, or new things that belong to the present time, which does not fit the context of the question at all.

"Outdated" refers to something that is no longer in style or practice. Although this can imply that something is old, it lacks the specific connotation of belonging to an earlier period that "archaic" conveys. Thus, "archaic" is the most precise term for it.