The part of a customer's account that is past its due date, read at one date : the amount itself (overdue), the same amount split by how long it has been late (four buckets), the part of it booked on doubtful accounts (doubtful), how late the oldest piece is (daysLate) and how many pieces make it up (numberOfDocuments). Nothing not yet due, nothing settled : a record that carries these figures says what is late, and only that.

🔑 The four buckets add up to overdue, and nothing else does. A piece sits in exactly one bucket, decided by its due date and the date the figures were read at. On a record read on 22 September :

Vialaret Joinery, read on 22 September :
  overdue1To30.value        3 100    due 21 days ago
  overdue31To60.value           0
  overdue61To90.value       5 400    due 62 days ago
  overdueOver90.value       2 650    due 98 days ago
  overdue.value            11 150    = 3 100 + 0 + 5 400 + 2 650
  doubtful.value            2 650    the oldest piece, booked on a doubtful account
  daysLate.value               98    the oldest due date passed
  numberOfDocuments             3    three pieces past due

⚠️ These figures are a snapshot, not a series. They are true at the date they were read and at no other : a payment recorded the next morning changes every one of them. A new reading replaces the previous one — nothing is ever added across time —, and the date it was read at travels with it, on whatever holds the trait (CustomerReceivables::$observationDate). For the same reason every measure carries a value and never a values : a snapshot has no months.

⚠️ Absent means « nothing of that kind », never zero. A record with no doubtful has nothing booked on a doubtful account ; a record with no daysLate has nothing past due — and then it should not exist at all, since these figures describe what is late. A zero would state that something was measured and found empty.

⚠️ daysLate does not add up. The amounts sum term by term across records — the overdue of a portfolio is the sum of its customers' —, but the days do not : ten customers ninety days late are not nine hundred days late. Whoever summarizes several records takes the largest, or leaves it out, and says which.

The amounts are those of the books, tax included, as the books state them : a salesperson collecting a payment needs the figure the customer actually owes.

They are not among the ten measures of HasTradingMeasures : those describe a year of trade, cut month by month ; this describes a debt at one date, with no month at all. A family composes this trait when it has such a debt to carry.

A property is typed null|array|ObservationSeries for the same reason as the ten measures : the union names a single class, so Reflection::hydrate() resolves a stored row on its own.