It's not necessarily subjective 'over time' - It doesn't even make sense to say something becomes subjective over time really unless you can think of a way where the evaluation is first not grounded in the opinion of persons and at a later time becomes grounded in the opinion of persons.
Situations can change and objective evaluations of moral value can remain a constant method. An example:
Situation 1. A tribe lives in a desert, there are low supplies of water. Because water is needed for hydration it is moral to promote an aversion to bathing. Bathing wastes the resources necessary for continuing to live, thus bathing is an immoral act despite the hygiene and aesthetic issues.
Situation 2. The same tribe travels a way and comes upon a great lake. Hydration is no longer a concern. The aversion to bathing that was previously morally good now results in an unnecessary level of poor heath and death from bad hygiene making it morally bad.
In each situation good and bad were determined by the same method (relative to any of greatest suffering, desires or survival) but the circumstances changed. It is still an objective moral theory.
Situations can change and objective evaluations of moral value can remain a constant method. An example:
Situation 1. A tribe lives in a desert, there are low supplies of water. Because water is needed for hydration it is moral to promote an aversion to bathing. Bathing wastes the resources necessary for continuing to live, thus bathing is an immoral act despite the hygiene and aesthetic issues.
Situation 2. The same tribe travels a way and comes upon a great lake. Hydration is no longer a concern. The aversion to bathing that was previously morally good now results in an unnecessary level of poor heath and death from bad hygiene making it morally bad.
In each situation good and bad were determined by the same method (relative to any of greatest suffering, desires or survival) but the circumstances changed. It is still an objective moral theory.
.
