Finding out how long something has existed in a relationship? (6)
I believe the answer is:
dating
I believe this is a double definition.
'finding out how long something has existed' is the first definition.
(as in dating an ancient artifact)
'in a relationship?' is the second definition.
(as in dating a partner)
(Other definitions for dating that I've seen before include "Going out with (a person)" , "Imposing time code or keeping social engagement" , "Taking out" , "Going steady" , "Seeing" .)