Today, I learned about the "Before Present" time scale, which is used in geology—specifically carbon dating. However, the "present" in the time scale is not defined as "whatever day the test took place," but rather specifically as January 1, 1950.
There are a few reasons for this. First, informally, with the amounts of time we're generally talking about for carbon dating, 65 years or so is usually well outside the realm of necessary precision. Second, that date is more or less when radiocarbon dating became practical. Third, and most interestingly, radiocarbon dating of stuff formed since 1950 is generally considered unreliable because the ratios of different carbon isotopes in the atmosphere were artificially altered by widespread testing of nuclear weapons in the '50s.
Note: Apologies for missing last night's post. Traveling has screwed with my habits. I take pride in bringing RM to you as close to daily as humanly possible, and I regret the mistake.