Reword the given text:
However, the Commission may grant a derogation, not exceeding one year, concerning the date of the first transmission of quarterly data from the first quarter of 1999 onwards, in so far as national statistical systems require major adaptations.
Where the national statistical systems require serious amendments, the Commission may exceptionally extend the period for the first delivery for the quarterly data for the period from the first quarter of 1999 for a period from one years at most.