The clock is read, never computed
A time written down is a time that was read off a clock; an estimate is said in words, never dressed up as a number.
A stamp taken once and carried forward ages silently. An hour later it is wrong, and nothing about it looks wrong: the digits are as crisp as they were when they were true. Whoever reads them next has no way to tell a read time from a remembered one, so the error passes unchallenged into the record and out of it into decisions.
The convention answers that: before a time is written into anything, the clock is read. Not inferred from when the work seemed to start, not added to the last stamp, not reconstructed afterward from memory. This is a house convention rather than a canon law — it was learned from agents that mislabelled times from a stamp that had gone quietly stale — and it is offered here as a practice that any body keeping a record will want.
The second half matters as much. An estimate said as a number acquires a precision it never had: about an hour becomes 14:30, and 14:30 becomes a commitment nobody made. So an estimate is said in words — before the turning, within the week, about two sessions' work — and it is only a date once someone has given a word for it.
Kept together, the two halves keep the record honest in the one place a record is easiest to corrupt without malice. The Game's own clock is kept the same way: the turnings are read off the sun, which no one owns and no one can quietly adjust.
Also called: times read from a clock, never computed Stands on: The Record · The Clock of the Game · Chronos Opens onto: Temporal markers apart from milestones · Honesty about time · Minutes are the one unit · Live log · Commitment · Provenance In play: table · Room Sources: The DNA of Heaven, Part XIII, the checks — is a time ever computed rather than read (Fable synthesis, unreviewed) · The Clock of the Game, in The Little Book of the Great Game (canon-grade) · lionsberg.wiki: The Clock of the Game Open: stated as a house convention carried into the seed's checks; it has no canon page of its own, and the Lexicon's entry for Chronos does not yet carry it.