The calendar knows in advance, the phone knows now

The flat in Merano stands empty most of the time. For it to be warm on arrival and secured on leaving, it needs to know two things: whether somebody is supposed to be there and whether somebody is there right now.

Those are two questions, and they need two sources.

My own rule from 21 August 2026 that settled the matter: “the calendar is updated at the weekend and will always be the authority for present and away.”

The division of labour

	Calendar	Phone tracking
answers	whether somebody should be there	whether somebody is there
can do so in advance	yes	no
knows visitors without a phone	yes	no
notices somebody staying longer	no	yes

The third row is the one explicitly noted as a weakness in the documentation: visitors without one of the two phones count as away for the tracker. That is not a fault of the service but its design — it only knows the devices it knows.

From that follows a split tied to each of the two events:

Arrival → the calendar alone. Pre-heating has to happen before arrival. No tracking can help with that, because at that moment nobody is there by definition.

Departure → the calendar decides that today is departure day. The tracking prevents the embarrassing case of arming the flat while somebody is still sitting in it.

Why there is evidence for that

The second row is not theoretical. Exactly that case occurred: the system stood armed for eleven hours while the house was full.

Nobody came to harm — but it is the reason the tracking is needed despite its known weakness. A calendar is an intention. It does not know that the return trip was postponed.

One source can see into the future and is wrong about the present. The other knows only the present and misses half of it.

Where the eight and ten hours come from

The two offsets look like arbitrary numbers. They are not — each has exactly one reason, and both reasons lie outside Home Assistant.

Arrival: minus eight hours

All-day events begin at 00:00. An event “Merano” on Saturday therefore begins in the night from Friday to Saturday.

Minus eight hours gives 16:00 the day before. And that is not a rounded figure but matches the condominium’s heating window, which according to the notice runs from 15:00 to 22:30.

So the pre-heating happens exactly when there is warm water in the circuit at all. Any earlier number would be an automation running against house rules.

Departure: plus ten hours

Here it is the other way round and more unpleasant: all-day events end at 00:00 of the departure day — that is, in the middle of the last night.

Without an offset the heating would be switched off at midnight while people are still asleep. Plus ten hours is the morning of the departure day.

That is why, with calendar triggers, you always have to work out what an all-day event technically means. The display says “Saturday to Monday”, but the event ends on Monday at 00:00 — and thus in practice on Sunday evening.

What it saves

Without all this, arriving would be a routine: turn up the heating, open the water, switch off the cameras, disarm the alarm. And leaving the same routine in reverse, with a chance of forgetting a step.

Now both hang on a calendar entry that is maintained anyway — because somebody has to know when they are travelling.

The cheapest automation is the one hanging off information that already exists for another reason.

What I’d suggest

1. Use the calendar for everything that has to happen beforehand. Pre-heating, filling the water line, switching off a dehumidifier — there is no alternative for that.
2. Use tracking for everything that must not happen while somebody is there. Arming, shutting off water, cutting power.
3. Work out when your all-day event technically starts and ends. 00:00 is rarely the moment the human means.
4. Look for the boundary condition outside your system. Here it was a notice in the stairwell with the heating window.
5. Write down your source’s weakness. “Visitors without a phone count as away” is in the file, and that is precisely why arming carries a second check.

So what?

Two sources for one question look like fussiness. They are the opposite: the question was two questions from the start, only nobody noticed as long as one source was supposed to answer both.

Eleven hours of an armed system in a full house is the result of mistaking a calendar for reality. A missed pre-heating would be the result of mistaking tracking for a diary.

And the two numbers that ended up in the file — eight and ten — come from no guide. One comes from a notice in the stairwell, the other from the fact that a calendar calls Sunday evening “Monday”.