18. The gross income that a worker derives annually from the employment that he carries on during the year for which the computation is made is determined on the basis of the gross income set forth in his contract of employment, to which are added the bonuses, premiums, gratuities, commissions, supplements for overtime, leaves if their cash value is not already included, profit sharing and the cash value of the personal use of an automobile or of a dwelling furnished by the employer.
Notwithstanding the foregoing, in the case of:(1) employment of fixed duration lasting less than 12 months, the gross income shall be increased for any period of the year, excluding 2 weeks, during which the worker does not carry on that employment, by 60% of the gross income that he would have derived from that employment had he carried it on during the period;
(2) employment that is seasonal, cyclical or on an on-call basis, the gross income is the income that the worker derived from the employment during the 12 months preceding the year for which the computation is made, and increased, for any period of the year, excluding 2 weeks, during which the worker did not carry on that employment, by 60% of the gross income that he would have derived from that employment had he carried it on during the period; where he is carrying on that employment for the first time, the gross income is the gross income that a worker in the same class derives from similar employment with the same employer or in the same region.