One person's numbers
Jun is a senior accountant at a shipping company in Manila on ₱80,000. His employer has always split the 13th month: ₱40,000 in early June, just before school opening, and the balance in December.
The June half is an advance on a year he hasn't finished yet. If he resigned at the end of August, his earned 13th month would be 8 × ₱80,000 ÷ 12 = ₱53,333.33, so his employer would owe him ₱13,333.33 more with final pay. Had he left at the end of March, he'd have earned only ₱20,000 — some companies then deduct the over-advance from final pay, which is why the split policy should be in writing.
Half in June, half in December: is that legal?
Yes. The 1987 guidelines say an employer may give half of the 13th month before the opening of the regular school year and the other half on or before 24 December. The frequency of payment is up to the employer, as long as the full amount is paid by the deadline.
At ₱80,000 the 13th month alone stays under the ceiling, so splitting it changes nothing for tax. If you also receive bonuses above ₱10,000, the excess is taxed at your 25% top rate.