Two-digit years

• Older systems and paper forms saved space by dropping the century.
• A spreadsheet display format showed two digits, and the file was exported as displayed.
• People type dates the short way when the century seems obvious to them at the time.

• Software picks a century using a cutoff rule, and different tools use different cutoffs, so the same file reads differently in each.
• Birth dates land in the future and ages come out negative, or a long-lived record appears a century younger than it is.
• Sorting and date ranges put the affected rows a hundred years away from their neighbors.

• Count values by length. A two-digit year makes the value shorter than a full date.
• After parsing, look for dates in the future in columns that should only hold the past, such as birth dates.
• Check the earliest and latest years. A gap of about a century between clusters is a sign.

• Use what the column means to limit the possible century. A birth date cannot be later than today, and a contract end date is rarely a century old.
• Compare against another date on the same row that the value must come before or after.
• Where both centuries remain plausible, send the row to a person instead of applying a cutoff.
• Write the result with a four-digit year so the question cannot come up again.

• Mixed date formatsOne column holds dates written in several different styles.
• Ambiguous day and month orderA date such as 03/04/2025 could mean March 4 or April 3, and nothing in the cell says which.
• Impossible datesA value is shaped like a date but names a day that does not exist or could not have happened.

TermsISO 8601, Parsing, Validation