How to convert mixed date formats in excel to iso 8601 in one pass
- Step 1Drop your Excel file — Upload an .xlsx or .csv with columns containing mixed date formats.
- Step 2Choose output format — Select ISO 8601, UK (dd/MM/yyyy), US (MM/dd/yyyy), or enter a custom date-fns format string.
- Step 3Download the standardized file — Every date is rewritten to the target format. A findings report lists any cells that could not be parsed.
Frequently asked questions
Does it handle dates stored as Excel serial numbers?+
Yes. SheetJS decodes Excel serial-number dates as JavaScript Date objects before formatting. No information is lost.
What happens to cells that aren't dates?+
Non-date cells are skipped. The tool only rewrites cells where a date can be confidently parsed.
Can I target specific columns only?+
In the current release all columns are scanned. Column-specific targeting is planned for the Pro roadmap.
Privacy first
Every JAD Excel tool runs entirely in your browser using SheetJS and ExcelJS. Your spreadsheets, formulas, and data never leave your device — verified by zero outbound network requests during processing.