The original title is: "Four silent XLSX export bugs in a Jira Forge app and how we fixed them"

Short summary

Four subtle XLSX export bugs discovered in a Jira Forge resource management app — none threw exceptions but all produced silently wrong output. Issues include date-only fields inheriting timezone-aware timestamps, numeric columns exported as text, and zero-rate billing producing invoices worth nothing. Each bug includes root cause analysis and a concrete fix, with advice to inspect raw XML rather than trusting rendered spreadsheets.

• •Date-only fields must never pass through timezone-aware types — use integer Excel serials instead
• •Numeric values formatted for display lose their cell type — explicitly cast on write
• •Zero-rate billing silently produces zero-cost invoices with no error or warning

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