Worked example
Paolo is a warehouse checker in Calamba, Laguna, on ₱20,000 a month with a 6-day week. His employer uses the 313-day factor, so his daily rate is ₱20,000 × 12 ÷ 313 = ₱766.77.
In April his father was hospitalised in Samar and Paolo took 4 days off without pay after using up his service incentive leave. His basic pay for the year is ₱240,000 − 4 × ₱766.77 = ₱236,932.91. His 13th month: ₱19,744.41, about ₱255.59 less than a coworker with perfect attendance. The paid SIL days themselves don't reduce anything — they were paid as salary.
How much do unpaid absences really cost?
Less than people fear. A day of leave without pay removes one day's pay from your basic salary, and the 13th month is one-twelfth of that — so each unpaid day costs you only about ₱63.90 of 13th month pay on ₱20,000. Tardiness and undertime deductions work the same way: they lower basic salary earned, and the 13th month follows.
What employers cannot do is withhold the whole 13th month as a penalty for absences, or make it conditional on “perfect attendance”. Attendance bonuses are separate and voluntary; the 13th month is mandatory.