From a stepwise-heating experiment to neutron-irradiated samples of whole rock and a plagioclase separate, we have determined an 40Ar-39Ar age of 4.26¿0.02 G.y. for the Apollo 17 troctolite 76535. This rock is believed to have formed as a result of very slow cooling at a depth of 10-40 km, and hence furnishes direct evidence that the moon was chemically zoned before 4.26 G.y. The age of the rock is associated with the Serenitatis Basin forming event. The age of formation of the Serenitatis Basin is then deduced at 4.26¿0.02 G.y. ago, about 300 million years before the Imbrium event. It appears that the major basin forming era on the near side of the moon lasted longer than 300 million years. The grouping of many lunea rock ages at 3.9-4.0 G.y. is most likely due to the fact that the Imbrium ejecta predominates all highland landing sites.