Such an account is necessary because the entitlement to vacation and to vacation pay during a calendar year is acquired on the basis of the work performed during the preceding calendar year.
So, a worker who was employed fulltime during the whole year 2012, will, in 2013, be entitled to
20 days of vacation, the normal salaryj for these 20 days of vacation (the simple vacation pay) and
to double pay for these 20 days (the double vacation pay).
Workers who, during the course of 2013, shift from fulltime employment to part-time employment, before having taken up all days of vacation for the year 2013, will not be able to pick up twenty days of vacation in full anymore (a day of vacation on which one has worked part-time, counts as a part-time day of vacation). This dissimilarity has to be compensated by the employer before the end of the year.
By means of illustration, imagine a person who, as from 1 October 2013, starts working half time, while at that particular moment having five days of vacation left. During the period covering 1 October up to 31 December 2013 he takes up the five remaining days of vacation. However, taking into account the fact that his has switched from fulltime to part-time employment, he has effectively only taken up 2.5 days of vacation during the last three months of the year. Thus, the worker is entitled to 2.5 additional days of vacation, days that have to be compensated by the employer before the end of the year by way of payment of the corresponding vacation pay.
This regime is quite comparable to the system of payment of the vacation pay of departure in the case of a termination of an employment contract, except for the fact that in this particular case the steady end of year bonus is not included in the annual remuneration serving as the basis for the calculation.
In addition, since this person has worked fulltime in 2013 for 9 months as well as three months halftime, he, in 2014, will be entitled to 15 full days of vacation and 5 halve days of vacation. However, assuming this person continues working halftime in 2014, he will, at the end of 2014, have taken only 20 halve days of vacation, corresponding to 10 full days. That difference has to compensated by the employer before the end of this year by payment of the corresponding vacation pay as well.
As from 1 January 2014, this second part will no longer have to be paid before the end of the year during which the reduction of working time took place. It will only have to paid during the following year. Consequently, as from next year, the employer will not have to pre-finance this part of the vacation pay anymore.
The example above concerns, for sake of convenience, a transfer from a fulltime to a halftime employment. However, to be noted, the account has to be made in the case of every reduction of working hours, whatever the size of the reduction.