CSV date format converter: one date format for a whole column
In short: Upload a list or paste the cells, pick the date column and the format you want. Slash, dash and dot dates, month names and ISO dates are read; day-first or month-first is worked out from the column or set by you; impossible dates are flagged and left as they were. Limit: Numeric dates where day and month could swap are read in the order found or chosen; check the count it shows.
Upload the file or paste the cells, choose the date column and the format you want, such as 2026-11-09. Dates written 9/11/2026, 9.11.2026, 9 Nov 2026 or Nov 9, 2026 are all read. If a value like 13/11/2026 shows the column is day first, the rest follow it; dates that are not real are flagged.
Price: this tool is free. Checking a whole email list is from $9/mo, and the first 100 checks are free with Google, no card. Check your list free nowSee pricing
Sample result
A mixed date column is read as day first and rewritten as 2026-11-13, 2026-11-09 and 2026-11-03.
- Dates rewritten
- 3
- Read as
- day first
- Could read either way
- 1
What this means for you
One impossible date is flagged and left alone, so the import does not guess.
- Slash, dash, dot and month-name dates
- Day-first or month-first worked out
- Impossible dates flagged
- Four output formats
- CSV
- Dates
- Import
By SimpleVerifier Team · Last updated · Runs in your browser; nothing you paste is sent to us
With the dates in order, check the addresses.
- Copy the Email column from the converted file, or use the check button under the result.
- Upload or paste it at /verify. Every address gets a live mailbox check, and the first 100 email checks are free with a Google sign-in, no card.
How do I put every date in a CSV column in the same format?
- Upload the .xlsx or CSV, or paste the cells with the header row first.
- Choose the date column (columns that hold dates are ticked for you) and the format you want.
- Leave the order on Work it out, or set day first or month first, and check the count of dates where the two readings differ.
- Download the converted file, or the list of dates it could not read, then import it.
Example
- You enter
- A Signed up column with
13/11/2026,9/11/2026,2026-10-05,Nov 3, 2026,31 Feb 2026and a blank cell, converted to year-month-day. - You get
- The column is read as day first, because 13/11/2026 can only be day first. Three dates are rewritten (2026-11-13, 2026-11-09 and 2026-11-03), one is already in that format, one is not a real date and is left as it was, and one cell is blank. One date, 9/11/2026, could be read either way.
- Limits
- Runs in your browser; nothing is uploaded. Files up to 25 MB.
How does it tell 3/4/2026 from 4/3/2026?
On its own it cannot: written with a number up to 12 in both places, the date reads two ways. The tool looks across the whole column. One value such as 13/11/2026 can only be day first, and one such as 11/13/2026 can only be month first, and the rest of the column follows. When nothing settles it, month first is assumed and the tool says so; choose the order yourself when you know the source.
Which date styles does it read?
Year-month-day with dashes, slashes or dots; day, month and year with slashes, dashes or dots; and dates with the month in words, such as 9 Nov 2026, Nov 9, 2026 or November 9th 2026. A time after the date is dropped. A two-digit year is read as 20 followed by the digits. Anything else, and any date that does not exist such as 31 February, is left as it was and listed.
Which format should I choose?
Year-month-day (2026-11-09) cannot be misread as day first or month first, so it is the safest to move between tools. Choose the form your importer asks for; if it does not say, try year-month-day first. The same cleaning belongs with the other column fixes in CSV column editor and the checks in CSV fill rate checker.
Who uses this tool, and for what?
- Sales ops
- Make a Created date or Last contacted column consistent before a CRM import or a cooldown filter.
- Cold email senders
- Put the dates from several exports in one format before merging them.
- Agencies
- Fix a client's mixed-format file before it goes into a sequencer.
What can’t this tool tell you?
- Numeric dates where day and month could swap are read in the order found or chosen; check the count it shows.
- Excel serial numbers and dates with non-English month names are not read.
- Only one column is converted per run.
- The time of day is dropped.
Common questions
- What about dates stored as Excel serial numbers?
- They are not read. A cell such as 46335 is listed as unreadable and left as it was; format the column as a date in the spreadsheet, or save the file as CSV first.
- Does it keep the time of day?
- No. The date is written without the time.
- Can it change one column and leave the others alone?
- Yes. Only the column you choose is rewritten; every other cell is copied across.
- Does it work on dates in another language?
- It reads English month names. Dates with the month in another language are listed as unreadable.
- What does it do with a blank cell?
- Leaves it blank and counts it.
- Is my file uploaded to SimpleVerifier?
- No. The file is read and rewritten in your browser.
A tidy list can still be full of dead addresses.
Extracting, deduplicating and syntax checks all work on the text of an address. None of them can tell whether the mailbox exists. Only the server that hosts it can answer that, through an SMTP check, and that is what the paid product does.
- Upload the cleaned file as it is: CSVs up to 100,000 rows
- A real SMTP mailbox check on every row, with catch-all detection
- Small list? Starter at $9/mo covers 2,500 emails a month, cancel anytime
Plans from $9 a month, cancel anytime, no contract. Sign in with Google and your first 100 email checks are free, no card. The free tools stay free either way.