A real-world example
Carlo is a civil engineer at a consulting firm in Makati, on ₱50,000 a month with a Monday-to-Friday schedule. He'd used his paid leave by May, then took 10 working days without pay in October for his sister's wedding in Toronto.
Daily rate on the 261-day factor: ₱50,000 × 12 ÷ 261 = ₱2,298.85. Ten unpaid days: ₱22,988.51 less basic salary. His 13th month: (₱600,000 − ₱22,988.51) ÷ 12 = ₱48,084.29 — about ₱1,915.71 less than if he'd stayed home.
Leave with pay vs leave without pay
Paid leave — vacation leave, sick leave, service incentive leave, solo-parent leave, paternity leave — is salary for those days, so it stays in your basic salary and doesn't reduce your 13th month. Leave without pay removes those days from what you earned.
The cash you get for unused leave at year-end is a different thing: DOLE specifically excludes the “cash equivalent of unused vacation and sick leave credits” from basic salary, so converting leave doesn't increase your 13th month either.