Irish buttered eggs - preserving eggs without refrigeration

Eggs were verboten during Lent:

This traditional Irish [from Cork] recipe—a method of preserving eggs from Lent through the Easter Sunday egg feast—requires just two ingredients: butter and a laying hen.
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The egg, still warm, was stolen from the hen and massaged in buttery hands. The butter seeped into the porous shell, acting as a barrier against air and thus preserving the egg for several months, or at least until the Easter Sunday egg buffet. It also allegedly lent the egg a subtle buttery flavor.

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Butter 8 freshly laid eggs, and then cook them 2 at a time in 2 week intervals. This way you should be able to compare 2, 4, 6 and 8 week old buttered eggs against eggs laid that very day and compare freshness.