OnlyBusy
Why a subscribed ICS feed does not count as Busy
Last updated 3 October 2026
Outlook and Google decide you are free by reading events on the calendar in front of them. A subscribed ICS feed is a separate calendar they poll on a delay, and that poll is often left out when they mark the hour free or busy.
How often a subscription refreshes
An ICS subscription is not a live event stream. The other calendar polls the URL. That refresh is typically every 6 to 24 hours.
A meeting you accepted this morning can still be missing from the subscription this afternoon.
Overlay versus an event on the calendar
Many clients draw a subscribed calendar as an overlay on top of the real calendar. The overlay is useful when a person is looking at the week.
An overlay is not the same object as an event that belongs to the calendar. Free/busy checks look at events.
Scheduling Assistant and Find a time
Scheduling Assistant and Find a time decide whether to offer an hour. They often ignore a subscribed calendar when they make that decision.
The hour stays open in the assistant even while the overlay, if you open it, shows a meeting.
When a feed is still the right tool
A feed is the right tool when someone wants to glance at your calendar and does not need their scheduler to treat the time as taken.
A downloaded ICS file is a third thing: one event a person imports. It is not a live subscription, and it does not update the other tenant when your week changes. The file generator is separate from this problem.
Questions
Why doesn't my ICS subscription show as busy in Outlook?
Scheduling Assistant often ignores a subscribed calendar. It reads events on the calendar in that tenant. A poll of an ICS URL is not one of those events.
How long does an ICS feed take to update?
A subscribed feed typically refreshes every 6 to 24 hours, so a new meeting can stay invisible for the rest of the day.
What should I use if the scheduler must see me as busy?
Write a native Busy event on that calendar. OnlyBusy does that with a private label and leaves the original title on the source calendar. The comparison with a feed is on the ICS versus OnlyBusy page.