Format Fidelity

Cell display text goes through a compensation layer that fixes a set of engine defects — each one verified against real behavior and locked by unit tests. This page documents what is restored exactly and what is approximated.

Builtin number formats: Excel's actual table

ECMA-376 defines builtin id 14 as mm-dd-yy, but Excel renders it as m/d/yyyy (en-US). The engine returns the ECMA string, and its loadFormatTable override cannot win (the builtin table always takes lookup precedence — a structural property of the engine). The preview therefore carries its own Excel-behavior table for the locale-dependent ids — 0–22 and 37–49 (14 → m/d/yyyy, 18 → h:mm AM/PM, 46 → [h]:mm:ss, …) — and resolves ids to format strings before formatting. Ids 23–36 (international-currency placeholders, rare in practice) keep the engine's ECMA strings instead of a hand-written behavior table.

Literal compensation in numeric sections

The engine drops literal characters inside numeric sections: ¥#,##0.00 renders as 1,234.50 (currency gone), #,##0.00;-#,##0.00 renders 1,234.50 for -1234.5 (sign gone), "neg"0 renders 5 (quoted text gone). The layer extracts safe edge literals (quoted runs, escapes, currency symbols, -, (, ), spaces), hands the numeric core to the engine with the absolute value, and re-joins — which also restores Excel's exact sign semantics (a negative section without - shows no sign, exactly like Excel).

Restored: currency prefixes/suffixes, units (0.0" kg"), text prefixes ("Total: "@), negative signs, accounting parentheses.

Approximated: literals interleaved between digits (phone-style 000-0000) and thousands-scaling commas (#,##0,) are still dropped; _ width-padding and * fill are intentionally dropped.

Dates and times: fully self-implemented

Two engine defects made a full rewrite necessary (the engine renders mm after h as month — 0.5 with h:mm shows 12:12 instead of 12:00 — and dddd as the day-of-month number):

• mm minute/month disambiguation by adjacency (after h, before s → minutes)
• Weekday names (ddd/dddd), month names (mmm/mmmm), 12-hour + AM/PM
• Elapsed durations [h]:mm:ss, [m]:ss, [s] (all broken in the engine)
• The bracket-less mm:ss (which the engine misreads as month:second) renders as time-of-day minutes:seconds — elapsed minutes is the bracketed [m]:ss semantics in Excel, and the two are kept distinct
• Fractional seconds (ss.0, builtin 47) round to the displayed precision
• date1904 workbooks: serials are shifted +1462 days before formatting
• Quoted literals (yyyy"年"m"月"d"日") and the 1900 ghost-day (serial 60) are handled

All time math uses UTC components (the industry-standard approach that avoids DST gaps), so a file renders identically in every timezone.

General

General is self-implemented: 15 significant digits with trailing zeros trimmed (the engine emits 16-digit float tails), uppercase TRUE/FALSE for booleans, raw strings for text.

Theme and indexed colors

The engine's read path drops theme/tint/indexed colors to null — that would lose the default font color (theme 1) and every themed fill on real-world files. The overlay layer unzips only styles.xml + theme1.xml (a few KB each), recovers the raw color specs, resolves them (theme palette with the 0/1・2/3 index swap, ECMA tint on HSL luminance, the indexed palette) and merges by array index. Explicit RGB colors take precedence; a failed overlay degrades gracefully to RGB-only.

Self-closed style elements (real-Excel id alignment)

Real Excel files write the default border as a self-closed <border/> (self-closed <font/>/<fill/> occur occasionally too). The engine's parser skips self-closed elements, so its fonts/fills/borders arrays come out one item short and every cellXfs id reference shifts: borderId=0 picks up the file's border #1, and the trailing reference dangles (verified with hand-built OOXML — the engine's own writer emits expanded forms, which is why round-trips through the exporter never trip this). The overlay layer therefore rebuilds all three collections from styles.xml in document order — self-closed entries included as empty items — keeping ids aligned with cellXfs exactly; the engine arrays serve only as the fallback when styles.xml itself cannot be read.

Colors from format codes

[Red] / [Blue] / [Color 3] section colors are extracted and applied as inline cell colors (the engine's plain formatCell mangles sections containing them entirely).