A real-world example
Rodel drives a company service van in Pasig. He earned ₱26,000 from January to June, then received a raise to ₱28,000 from 1 July 2026.
His basic salary for the year: 6 × ₱26,000 + 6 × ₱28,000 = ₱324,000. Divided by 12, his 13th month is ₱27,000 — not ₱28,000. Several of his coworkers expected the new rate to apply to the whole year; it doesn't, because the law looks at basic salary earned during the year, month by month.
Mid-year raises and back pay
If a wage order or a raise is applied retroactively — say, approved in October but effective from July — the salary differential you receive for July to September is basic salary for those months, so it does increase your 13th month. That matters in 2026 in Metro Manila, where the minimum wage changed under Wage Order NCR-28 from 26 September.
Promotions work the same way. Moving from ₱28,000 to ₱32,000 in October would give (9 × ₱28,000 + 3 × ₱32,000) ÷ 12 = ₱29,000.