This study places the recently excavated ninth-century A.D. four-part calendrical notations at Xultun (Lunar Table, Ring Number, Long Count, and Multiplication Table) in the context of both Classic monumental inscriptions and astronomical knowledge in the Postclassic Dresden Codex. We demonstrate that the Lunar Table employed a formula attributed to Palenque and that it could have been used as a device to determine precisely where to break the sequence of alternating 29- and 30-day months one finds on dated monuments. All four categories found at Xultun appear in the Dresden Codex. The Ring Number, which bridges a date in the deep past with one in the recently completed era, is a perfect fit with one of the most fundamental Dresden eclipse cycles. Our analysis of glyphs accompanying the Long Count date enables us to place candidate eclipses, especially one that corresponds with a conjunction of Mars, in real time. We argue that the large multiples were extracted from, or prepared for, warning tables like the Dresden eclipse table, and we demonstrate why such tables must have existed well before the Xultun inscriptions. Thus, while the writings in the Dresden manuscript constitute a finished product, the writing on the wall of residential Structure 10K-2 is more akin to what one might find in an astronomer's notebook.