How do I represent the age/staleness of composite data?

Hello UX!
quick question:

A system has a quantity that is the product of two others: C = A * B

Imagine A is an amount in Euro, B is the exchange rate to USD/EUR and C is the equivalent amount in USD.

Now, the values of A and B also have a timestamp, when they were refreshed last.

A might have been updated 3 days ago, and B a month ago.

1. How do I define the "staleness" of C?
2. Is it better to show a timestamp or some sort of fuzzy indicator?

thanks!