What “12 calendar months” means, and why it is not 365 days

The rule, in one sentence

An inspection is good for the rest of the month it was done in, plus that many whole calendar months. 14 CFR 91.409(a) puts it as an aircraft having had an annual inspection “within the preceding 12 calendar months”.

Which inspections work this way

The 100 hour inspection under 91.409(b) is not on this list, because it counts hours in service rather than the calendar. Many recurring Airworthiness Directives are on it, usually written as an hour figure and a calendar figure with whichever comes first binding.

Why 31 January plus one month has no answer

Counting forward from the day is what makes the arithmetic awkward: there is no 31 February, so software either clamps to the 28th, rolls into March, or picks something else. All three are wrong for different dates.

Counting to the end of the month is always defined. One calendar month after January is February, and February always has a last day. There is no leap-year branch and no clamping, which is a good sign that it is the right way to model it.

The cost of the safe mistake

Tracking a calendar month interval as a day count always errs early. That sounds harmless, and it is the reason it survives for years: nothing breaks, nobody is unairworthy, and no inspector ever mentions it.

What it does is take an aeroplane off the line while it is still perfectly legal to fly. Average it across a fleet of ten with four calendar inspections apiece and the school is losing real availability every year to arithmetic. It is worth an afternoon to check how yours are set.

How AerScheduler handles it

Calendar months are a real interval, not a day count. Pick days, weeks or calendar months when you set an inspection up, and the due date lands on the last day of the month, worked out on your school's own time zone. The standard airworthiness presets use calendar months where the regulation does.

An interval is also written back to you in the unit it is stored in. A rule set to 365 days reads “365 days” rather than being rounded to twelve months, which is how you find the ones worth converting.

Related

Airworthiness Directive tracking covers the recurring directives that use these intervals. Add your own inspection is the step-by-step version.

Questions

What does 12 calendar months mean for an annual inspection?

The inspection is good for the rest of the month it was done in, plus twelve whole calendar months. An annual signed on any day in February 2026 is good through 28 February 2027. Signed on the 1st or the 28th of February, the expiry is the same date, which is what makes it a calendar month rather than an anniversary.

Is 12 calendar months the same as 365 days?

No, and the gap is up to a month. 365 days from 15 February 2026 is 15 February 2027, but the aircraft is legally airworthy until 28 February 2027. The difference depends on which day of which month the work was signed, and it is largest when the inspection falls early in a long month.

Which inspections run on calendar months?

The annual under 14 CFR 91.409(a) and the ELT under 91.207(d) are 12 calendar months. The transponder check under 91.413 and the static system and altimeter check under 91.411 are 24 calendar months. The 100 hour under 91.409(b) is not a calendar interval at all: it counts hours in service. A great many recurring Airworthiness Directives are also written in calendar months, often as an interval alongside an hour figure.

Which day of the month should I record the inspection on?

The day the work was actually signed. It does not change the expiry, since every day in the same month gives the same answer, but the record has to say what happened. Recording it as the last day of the month to make the arithmetic tidy puts a false date on a maintenance record.

What does it cost to track a calendar month interval as 365 days?

Roughly two weeks of availability per aircraft per year on average, and up to a month in the worst case, for every inspection tracked that way. On a fleet of ten with four calendar inspections each, that is a lot of aeroplane sitting on the ramp perfectly legal to fly. It is the safe direction to be wrong in, which is precisely why it goes unnoticed for years.

What about 31 January plus one calendar month?

There is no 31 February, which is why counting from the day never quite works. Counting to the end of the month always does: one calendar month after January is February, and the interval expires on 28 or 29 February depending on the year. That is also why the arithmetic needs no special case for leap years.

Does the time zone matter?

It does, in one narrow case that is easy to get wrong. Work signed at six in the evening on 31 January in Denver is stored as 1 February in UTC. Reading the month off the wrong clock moves the expiry a whole month later, which is the one direction this arithmetic must never be wrong in. AerScheduler reads the month on the school's own time zone for exactly this reason.

Can I set inspections in weeks?

Yes. A week is exactly seven days and always will be, so weeks and days are the same thing to the arithmetic. AerScheduler offers days, weeks and calendar months, and stores a week interval as days while reading it back to you as weeks.