Read Numbers 19:12 Using Other Translations
He shall purify himself with it on the third day, and on the seventh day he shall be clean: but if he purify not himself the third day, then the seventh day he shall not be clean.
He shall cleanse himself with the water on the third day and on the seventh day, and so be clean. But if he does not cleanse himself on the third day and on the seventh day, he will not become clean.
They must purify themselves on the third and seventh days with the water of purification; then they will be purified. But if they do not do this on the third and seventh days, they will continue to be unclean even after the seventh day.