Scientists use observational science to measure the amount of a daughter element within a rock sample and to determine the present observable decay rate of the parent element.
Since we did not observe the initial conditions when the hourglass time started, we must make assumptions.
All three of these assumptions can affect our time calculations.
If we walk into a room and observe an hourglass with sand at the top and sand at the bottom, we could calculate how long the hourglass has been running.
By estimating how fast the sand is falling and measuring the amount of sand at the bottom, we could calculate how much time has elapsed since the hourglass was turned over.
That is, billion-year half-lives can be measured statistically in just hours of time.