How does it tell which column is a date column?

It tries to parse every cell in each column. If at least 60 percent of the non-empty cells parse as a date, the column is treated as a date column. This avoids accidentally rewriting ID or amount columns that occasionally look date-like.

How does it resolve DD/MM/YYYY versus MM/DD/YYYY?

When a value has a part greater than 12 (like 25/03/2024) the order is unambiguous, so those cells vote for the column's format. The majority vote then decides how to read the genuinely ambiguous cells like 03/04/2024, which are flagged with a warning.

What is ISO 8601 and why use it?

ISO 8601 writes dates as YYYY-MM-DD, for example 2024-03-12. It is unambiguous worldwide, sorts correctly as plain text, and is the format databases and spreadsheets handle most reliably, which is why it is the normalisation target.

What happens to cells it cannot parse?

Unparseable cells are left exactly as they were and flagged with a warning marker so you can fix them manually. The tool never silently corrupts a value it is unsure about.

Is my CSV uploaded anywhere?

No. Parsing and normalisation run entirely in your browser. Your data never leaves your device, so it is safe for customer lists and other sensitive exports.

CSV Date Format Normalizer

Spreadsheets and exports often mix date formats in the same column — 12/03/2024 next to 2024-03-12 next to 3 Apr 2024. That breaks sorting, imports, and analysis. This CSV date normalizer detects the date columns and rewrites every value to clean ISO 8601 (YYYY-MM-DD), entirely in your browser.

How it works

Parse the CSV with a small parser that respects quoted fields and embedded commas.
Classify columns. Each column is scanned; if most of its non-empty cells parse as dates, it is treated as a date column. Non-date columns are passed through untouched.
Resolve day-month order. The hardest part of date cleaning is telling DD/MM from MM/DD. The tool uses unambiguous cells — any value with a part greater than 12, like 25/03/2024 — to vote on the column’s true order, then applies that decision to the ambiguous cells.
Rewrite to ISO 8601. Every recognised date becomes YYYY-MM-DD, validated against real month lengths and leap years so impossible dates are rejected.

Tips and notes

Cells that stay ambiguous (both parts ≤ 12 with no majority signal) and cells that cannot be parsed are marked with a ⚠ so you can review them — nothing is silently guessed wrong.
Supported inputs include YYYY-MM-DD, DD/MM/YYYY, MM/DD/YYYY, dotted DD.MM.YYYY, and textual months like 3 Apr 2024 or April 3, 2024.
Leap years are handled correctly, so 29/02/2024 is accepted but 29/02/2023 is flagged as invalid.
The whole process is local, making it safe for customer lists, invoices, and other confidential CSVs.