What dictates my service intervals - mileage or time?

My Subaru Forester's service schedule is every 12,000 miles or once a year. My last service after three years should have been the 36,000 mile service but, since I had only covered 19,356 miles, I chose to have the 24,000 mile service. What should be the determining factor for the services, the mileage or the annual anniversary irrespective of mileage?
Every car needs a service and an oil and filter change at least once a year. A lot of models with chain cam engines and long service intervals are now suffering stretched or broken timing chains as a direct result of oil contamination.
Answered by Honest John on

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