Excel & Google Sheets · Reference

Understand the date behind the cell.

Start with the symptom: a date that sorts like text, a swapped month and day, a four-year shift, or two matching labels that compare unequal.

Is this a date, a number, or text?

Diagnose the stored value before changing its appearance.

A plausible-looking result can still be the wrong date.

Check Excel’s workbook date system before adding an offset.

Separate a calendar day, a time of day, and an instant.

Use a small date-focused acceptance test before the real file.

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