(a) 30-day period. The flat rate of duty shall not apply to a person who has used the provision within the 30-day period immediately prior to his arrival in the United States. The date of the person's last arrival on which he declared articles for which the flat rate of duty was applicable shall be considered the date that rate was last used.

(b) Computation of time. The 30-day period immediately prior to the person's arrival in the United States shall be computed by excluding the day of arrival and counting backward 30 days.

(c) Remainder not applicable to subsequent journey. A person who has received a flat rate of duty allowance of less than $1,000 in connection with his return from one journey is not entitled to apply the remainder to articles acquired abroad on a subsequent journey.