How absolute are absolutes?

When modeling a system that needs to last a long time or be used by a very large number of users - you can run into problems with absolutes. It turns out that very few aren't subject to occasional change or are otherwise subject to interpretation:

• weight - the kilogram is based on a reference original as well as forty copies. However, simply touch or dropping one can make subtle differences in weight - and in 2009 it was discovered that none of these weight exactly the same amount - each are off by a handful of micrograms. Some of these have grown steadily lighter in weight over time.
  
• length - the meter was initially defined in Paris in 1790 as the length of a pendulum with a half period of one second. In 1791 it was changed to be one millionth of the earth's merridan along a quadrant running through Paris. In 1793 the previous definition was discovered to be long by one fifth of a millimeter. Between 1795 and 1799 two reference bars were created (first brass then platinum). Then between 1889 and 2002 the definition was changed five times to consider temperature, gravity, and atmosphere, to be based on atomic decay, then be based on a the speed of light over a fraction of a second.
  
• eye color - is affected by lighting, contact lenses, a person can have two different colors, or a different color on the outside vs the inside of the iris. And then you've got hazel - which there's no standard for.
  
• planets in the solar system - even the definition of 'planet' has changed often. Two thousand years ago it was 'a body that moves across the sky', then was upgraded to unwandering orbs that move across the sky and keep their relative position and distance at which time 5-7 qualified. In the fifteenth century it became a body that orbits the earth. In the sixteenth century it was a body that orbits the sun. In the nineteenth, a large body that orbits the sun at which time between 8 and 23 bodies qualified. The latest definition from the IAU in 2005 states that 'A "planet" is a celestial body that: (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit. And now Pluto is out.
• time - calendars have changed over time, the definition of the second has changed (initially 1/60th of a minute, in 1956 it was based on a fraction of the earth's rotation around the sun, in 1967 it was changed to be based on the rate that caesium-133 decays. In 1980 and 1997 this last definition was amended to consider altitude and temperature. Additionally, many facts are based in a point in time - but are different at other points in time (a person's height, weight, marital status, etc).
• etc - laws, social norms, races, categories of any field, it all changes given enough time or communities. Except for math.
  

So, what's the real practical significance of all this? Well, for most systems development little really. But for those projects that consider data over a span of centuries, to compare data across many geographies, or that need to become a piece of foundational infrastructure - there may be advantages to modeling certain facts as relative rather than absolute values. Given the complexity and performance costs of modeling relative values this isn't something that I'd recommend automatically doing, but definitely something to keep in mind for projects of a certain type.