When Aplum saw the moon set on the western horizon at sunset he knew that a new month was beginning. The Babylonian calendar had 12 months of 29 or 30 days, a total of 354 days.
As a year lasts 365 days, the Babylonian calendar always started 11 days too early. Over time it no longer matched the seasons. Aplum's job was to advise the king when an extra month needed to be added to the calendar to bring it back in line with the seasons.