You need to know about two filings
The first is your Estimated Chargeable Income (ECI), due within three months of your financial year end. It's a preliminary estimate IRAS uses to raise a provisional assessment, filed before your actual return.
The second is your annual Corporate Income Tax Return, which is due by 30 November every year regardless of your financial year end. Which form you use amongst Form C-S, Form C-S (lite) & Form C, depends on your company's size. All companies need to file, including dormant ones, unless specifically exempted by IRAS.
Miss either deadline and you're looking at penalties rising for repeat offenses.
Beyond filing: planning, advisory, and staying complied
Filing accurately is the baseline. The more valuable part of working with a tax advisor is usually everything around it.
Tax planning looks at your income, expenses, and structure ahead of time, not after the year's already closed, to make sure you're set up to claim what you're entitled to.
Tax advisory covers the bigger decisions: how a business restructure affects your tax position, what a cross-border transaction actually costs you after tax, how to structure a new investment or expansion efficiently, and what corporate acquisitions mean for your liabilities.
Tax compliance support is the ongoing side of things, keeping a calendar of your filing deadlines, reviewing records regularly rather than scrambling once a year, and keeping you current on regulatory changes that might actually affect your business.