A date system change is a workbook-wide change

A workbook can keep every stored numeric cell and every formula exactly the
same while changing how date serials are interpreted. That is easy to miss in a
cell-by-cell review, and costly to discover after a report shifts by years.

FormulaFence 0.220.0
adds FF117, a high-severity finding for a normalized change to the workbook's
serial-date controls. It watches workbookPr/@date1904 and
workbookPr/@dateCompatibility directly in raw OOXML, before ordinary readers
turn a date-formatted number into an application value.

Watch the control, not a guessed outcome

Microsoft's WorkbookProperties documentation
defines the controls: date1904 selects the 1900 or 1904 compatibility base,
while dateCompatibility determines whether that compatibility base is in use.
The documented defaults matter: omitted date1904 is false and omitted
dateCompatibility is true. FormulaFence normalizes those defaults, so an
omitted declaration and an explicit documented default do not make CI noisy.

The rule is observational. It does not calculate formulas, convert serials to
dates, predict a client display, or claim that a saved result is current. It
reports the stored control transition so a reviewer can decide what it means for
the workbook's actual consumers.

A policy boundary people can read

Profiles and FF117 details expose only normalized Boolean state, whether
dateCompatibility was explicitly declared, and an unrecognized-control count.
Invalid, duplicate, or unreadable metadata becomes an explicit coverage warning
rather than a guessed epoch.

To block the change in CI:

version: 1
rules:
  no_workbook_date_system_changes: true

That turns the observed FF117 into FFP117.

Evidence before the claim

The release fixture keeps a raw numeric serial, date number format, ordinary
formula consumer, and downstream dashboard consumer fixed. Only
xl/workbook.xml changes: date1904 moves from false to true while
dateCompatibility remains true. Separate cases prove that documented default
spelling stays quiet, a compatibility-mode change is detected independently,
and malformed controls remain visibly unassessed.

FormulaFence 0.220.0 passed 1,589 tests, hosted CI, package metadata checks,
and fresh wheel/source-distribution installs. Read the canonical release note
for the full boundary, validation record, and install command. FormulaFence is
MIT-licensed and available on GitHub.