Forgive me, It as been a long time since I dealt with decay of atoms. These days I am just trying to make sure that welders are welding within their qualifications.

No worries...that's more difficult than navigating a periodic table anyway.
Co58 has a half-life of 70 days. Co60 (we all know)...5.3 years.
So, if you analyze a sample of something and it has Co58, that isotope was "made" in the last 300 days (5 1/2 lives) or so.
This concept makes for an interesting discussion when someone (not knowing this concept) tries to convince you (any Rad Engineer out there) that this contamination residue has been there for over a year (read: before I was in this job).
You rad engineers and health physicists out there have ALL the cool toys.
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