(a) 30-day period. The $800 or $1,600 exemption for articles acquired abroad will not be granted to a returning resident who has taken advantage of such exemption within the 30-day period immediately preceding his return to the United States. The date of the returning resident's latest prior arrival on which he declared articles acquired abroad for allowance of the $800 or $1,600 exemption will be deemed the date he took advantage of the applicable exemption.

(b) Computation of time. The 30-day period immediately preceding the resident's return will be computed by excluding the day of arrival and counting backward 30 days. For example, in the case of an arrival on May 28, the resident would not be entitled to the $800 or $1,600 exemption if he had taken advantage of such exemption on or after the preceding April 28.