How to use the ambiguous date column resolver
- Paste the dates from one column, one per line, with a four digit year, using /, . or - between the parts.
- Choose what to do when nothing in the column proves the order: month first, day first or leave them unresolved.
- Read the headline to see which order the column uses and why.
- Copy the table or the CSV, and check the rows marked ambiguous.
Formula
first part greater than 12: the day comes first; second part greater than 12: the month comes first
both parts 12 or less and different: ambiguous; equal: the same either way
column order = the order proved by any row; otherwise your fallback
One row that cannot be a month in the first position proves the whole column is day first, and a row that cannot be a month in the second position proves it is month first. A column with both is mixed.
A column of seven dates
The example column has 13/04/2026, 21/07/2026 and 31/02/2026, whose first parts are above 12, so the column is day first. 03/04/2026, 05/06/2026 and 08/09/2026 are ambiguous on their own and are converted as day first to 2026-04-03, 2026-06-05 and 2026-09-08. 02/02/2026 is the same either way.
The date 31/02/2026 gives the day first order but is not a real calendar date, so it is not converted. With month first as the fallback and no row above 12, the same ambiguous dates would convert to 2026-03-04, 2026-05-06 and 2026-08-09.
Evidence is not proof
A column often comes from one source that uses one order. If a single row shows the order, the other rows almost certainly share it. But a file that merges two sources can mix both orders, and then the evidence from other rows says little about an ambiguous one.
The page shows the number of rows proved day first, proved month first and ambiguous so you can see the evidence before you trust the result. A mixed column is flagged.
What is converted
Dates are written as year-month-day, which sorts correctly and has only one reading. Two digit years are refused, since the century is unknown, and impossible dates such as April 31 are marked instead of being fixed. Dates already in year-month-day form pass through unchanged.
To translate format patterns between systems, use the Date Format String Translator.
Assumptions and limits
- Each date has a four digit year and two numeric parts separated by /, . or -.
- Evidence from other rows is a hint and the choice for ambiguous rows can still be wrong.
- Month names and times are not read.
- Up to 2,000 dates are read.
- Nothing you type is stored or sent anywhere.
Frequently asked questions
Why does one date decide the whole column?
Because a column normally comes from one source with one convention. It is not proof, which is why the page shows the evidence counts and flags mixed columns.
What if I know the order already?
Choose it as the fallback. If a row contradicts it, the page converts that row by its own evidence and shows the difference.
Why are two digit years refused?
The century cannot be known from the text alone, and guessing wrongly would change the date by a hundred years.
What does Same either way mean?
The two numbers are equal, like 02/02/2026, so day first and month first give the same date and no choice is needed.