Why Subscribed Calendars Stop Updating (And Why It Isn’t Your Fault)

Most developers who work with calendars eventually hit the same strange bug. Someone subscribes to your ICS feed. Everything looks fine. Then a day or two later nothing updates. Events go missing. Times drift. Outlook users swear the feed is frozen. Google polls whenever it feels like it. Apple refreshes until it doesn’t.

You debug your code. You check the ICS. You regenerate the file. You wonder if you broke SEQUENCE again.

None of it fixes the problem.

The truth is simple but not obvious. Subscribed calendars do not behave like synced calendars. They are not real time. They are not predictable. Every client polls on its own schedule, caches aggressively, and decides for itself whether a feed is worth refreshing.

A few examples:

• Outlook sometimes polls once a day and sometimes barely at all.
• Google may not fetch for hours and sometimes days.
• Apple refreshes more often, but only when its own heuristics agree.
• Some clients do not re-fetch unless the URL changes.
• Large feeds get throttled quietly.

Your users assume the feed is live. The calendar clients do not share that assumption.

This is why so many subscription bugs look random. The ICS is valid. The update logic is correct. The client simply has not refreshed yet, and you have no way to force it. The only reliable tricks are versioned URLs, tight caching headers and smaller feed windows. Everything else is guesswork.

This is also one of the reasons I am building Synara. It handles event consistency upstream so you do not rely on each client’s refresh behaviour to interpret updates correctly. It does not fix subscription polling, nobody can, but it removes the ICS headaches that usually get blamed for it.

If you have dealt with subscription bugs or had to explain to users why their calendars look frozen I would love to hear your experience. Developers swap war stories about ICS, but subscription behaviour deserves its own category.