Employee bonuses are taxed in all the months they refer to, not only in the month they are paid.
Employers often use bonus system in addition to salary remuneration to increase its employee performance. Bonuses can be discretionary (not guaranteed) or nondiscretionary (guaranteed as shown in your employment contract). Discretionary bonuses are paid on a performance-based evaluation. The bonus can vary from 0% to 200% of the salary, depending on the degree of achievement of the goals – the employer does not guarantee a minimum amount of the premium, nor does it owe advance payments during the year.
Since the last changes of Social Security Code from August 2023, earned bonus income should be taxed retrospectively in the months in which the work was performed, not just in the last month. For instance: the annual bonus granted to a employee in the end of the year for the results achieved during this period must be taxed not in December but rather in all the months of the year.
Art. 6, para. 2 of SSC /in force from 1.08.2023/
“The income on which insurance contributions are due includes all remuneration, including accrued and unpaid or unaccrued and other income from employment. Remuneration relating to work done for the past time as quarter, annual and other bonuses is allocated to the days worked during which the work was done.“