The Board held that, if the seven year period of continuous residence stops for 240A(a) cancellation of removal, it does not restart again based merely on a departure from and reentry to the U.S. A key fact in this case, however, is that the conviction that stopped Nelson's period of continuous residence also made him inadmissible at the time of his reentry to the U.S. There also was no claim that he obtained a waiver of that inadmissibility. The Board reserved deciding whether the result would have been the same if he had been readmitted with a waiver. The Board also attached some significance to the fact that the conviction was charged as a basis for removability (in addition to other grounds of removability), although it did not explain the exact relevance of this fact.
Read the opinion at http://www.justice.gov/eoir/vll/intdec/vol25/3704.pdf