The two 24-hour deadlines
There are two deadlines, not one. You must report a booking within 24 hours of it being made or cancelled. You must then report the guests within 24 hours of them arriving. Most guidance only mentions the second, which is why many owners are quietly missing the first.
Ask this question online and you will get two confident, contradictory answers from people who have been letting property in Spain for years.
They are not being careless. The rules set two separate deadlines, and most summaries only describe one of them.
What the rule actually says
Two words in there do a lot of work.
Respectively. Each of those moments starts its own clock. You do not choose between them.
Cancellation. If a booking is cancelled, that is itself something you have to report.
There is also a phrase worth noticing at the start: immediately. Twenty-four hours is the outer limit, not the target.
Clock one: when the booking is made
This starts the moment a guest books. It also starts if they cancel. It applies whether the booking came through a website or straight from the guest.
This is the deadline almost nobody knows about.
"But I don't have their passport details when they book"
This is the most common objection, and it comes from a misunderstanding about what each deadline is asking for.
The first report is about the booking: the reference, the dates, the property, how it was paid for. It is not asking who is coming.
The passport details, dates of birth and nationalities belong to the second report. Nobody expects you to have them the moment someone clicks "book".
Clock two: when the guests arrive
This starts when the stay begins. In practice, at check-in. This is the report with the guests' personal details in it.
Before either of them: the property itself
Separately from both deadlines, there is a one-off report about your business and your property. It has to be done before you start letting.
If the website is down
The clock keeps running. There is no provision anywhere for the deadline pausing because the government's own system is unavailable — and SES.Hospedajes has no status page you can point to afterwards.
What being late actually costs
Being late is a minor offence. The fine is €100 to €600.
That is a very different thing from not filing at all, which is a serious offence and runs from €601 to €30,000.
One more thing worth knowing: these fines expire — six months for the minor ones, a year for the serious ones. There is more on all of this, including the six-figure numbers you will see quoted, under the fines.