12 They must rededicate themselves to the LORD for the same period of dedication and must bring a year-old male lamb as a guilt offering. The previous days do not count, because they became defiled during their period of dedication.

Read Numbers 6:12 Using Other Translations

And he shall consecrate unto the LORD the days of his separation, and shall bring a lamb of the first year for a trespass offering: but the days that were before shall be lost, because his separation was defiled.
and separate himself to the LORD for the days of his separation and bring a male lamb a year old for a guilt offering. But the previous period shall be void, because his separation was defiled.
The days of their vow that were completed before their defilement no longer count. They must rededicate themselves to the LORD as a Nazirite for the full term of their vow, and each must bring a one-year-old male lamb for a guilt offering.