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A counter delta is booked at the reading that closes it. That is right at the reporting cadence -- a monthly series books December against the 1 January reading, which is what the reference sheet does -- and wrong after an outage. Observed on Solar 1: 78 days of generation arrived as one July row, leaving June looking like the array was switched off. An interval containing two or more complete calendar months is now divided across them in proportion to elapsed time. The meter recorded a total, not a shape, so every row a split produces is marked Estimated. The sum is exact: the final segment absorbs the rounding remainder, so a split never creates or destroys energy. Counting whole months *contained* rather than boundaries *crossed* is what makes the rule safe. A monthly series contains exactly one whole month per interval and is untouched, so the golden fixtures keep measuring the normalizer rather than the splitter; and a reading landing hours late cannot tip the rule and hand the new month a sliver. GapSplittingIsInertOnFixturesTests asserts the rule declines to fire on every reference interval, so this cannot drift into the oracle unnoticed. Not apportioned: swap and reset amounts (explicit corrections booked at their event -- apportioning one would rewrite a number the operator supplied), a rejected decrease, and a zero delta, which would otherwise fan out into rows that say nothing. Segments are stamped at their end, keeping the existing convention that a row records the period ending at its timestamp -- so nothing shifts relative to how unsplit intervals are already labelled. Claude-Session: https://claude.ai/code/session_01V6joyergfvVLFEizH1hJLd