CSV Date Format Converter

About the CSV Date Format Converter

Paste a list of dates or open a CSV or Excel file, and write every date in the format you need: ISO 8601 (2024-03-04), day first (04/03/2024), month first (03/04/2024), with month names (4 March 2024), as an RFC 3339 timestamp with its time zone, as a Unix timestamp, or as an Excel serial number in the 1900 or 1904 date system. The tool reads dates written in all those ways, also with times, AM and PM, time zones, weekdays, two-digit years and month names in other languages.

A date like 03/04/2024 is 3 April in most of the world and March 4 in the United States, and nothing in the value says which. So the tool never guesses: each column’s order comes from its own values that can be read only one way (25/03/2024 is day first), and when a column has none, its ambiguous values are flagged and left as they are until you choose. Real dates in Excel cells are read as the dates they are. Every value that is not converted is listed with its row and the reason. Everything runs in your browser; nothing is uploaded.

Common uses

• Turning DD/MM/YYYY dates from an Indian, British or European system into ISO 8601 for a database or an API.
• Fixing a CSV whose dates Excel shows as numbers like 45355, or a 1904-system workbook from an old Mac whose dates are four years off.
• Converting Unix timestamps in a log or analytics export into readable dates in your time zone, or the reverse.
• Finding the rows of a file where day-first and month-first dates are mixed up before a report is built on them.
• Writing dates with month names for a letter, an invoice list or a mail merge.

Day first or month first: never guessed

Numeric dates whose first two numbers are both 12 or less can be read two ways. The tool counts, per column, the values only one order allows: a column with 25/03/2024 is day first; one with 03/25/2024 is month first. Values like 03/04/2024 in such a column are read in that order. A column with no such value is undecided: its dates are flagged as “can be read day first or month first” and left as they are until you choose the order for that column (or for all columns). A column that has both kinds is mixed: each value one order allows is read that way, and the others are flagged. Year-first dates (2024/03/04), ISO 8601 and month names are never ambiguous.

Excel serial numbers

Excel stores a date as the number of days since its starting point. In the 1900 date system, the default of Excel for Windows and of current Excel for Mac, 1 is 1 January 1900; in the 1904 date system of earlier Excel for Mac, 0 is 1 January 1904, and the same date is 1,462 days lower (Microsoft: date systems in Excel). Excel also counts a 29 February 1900 that never existed, kept for compatibility with Lotus 1-2-3 (Microsoft): serial 60 is that day, so the tool flags it rather than shifting it. A fraction is the time of day: .5 is noon. When you download Excel, converted dates can be written as real date cells.

Two-digit years

Two-digit years follow Excel’s rule by default: 00 to 29 are 2000 to 2029, and 30 to 99 are 1930 to 1999 (Microsoft: how Excel works with two-digit year numbers). Change the cut-off under How to read them for files with other conventions, such as birth dates.

Time zones

Values with a zone (2024-03-04T10:15:30+05:30, …Z, GMT+5:30) and Unix timestamps are moments in time; values without one are wall-clock times. Values without a time zone are in gives the zone such times were recorded in; Write times in converts every moment to another zone, using the time zone rules your browser carries, daylight saving time included. A time that does not exist in a zone (the hour the clocks jump forward) is moved forward and flagged; a time that happens twice (the hour the clocks go back) takes the first and is flagged. Formats that need a zone — RFC 3339 and Unix time — read values without one as UTC, and say so. Abbreviations such as IST or CST name several zones, so values that use them are flagged.

Formats you can write

• ISO 8601: 2024-03-04, 2024-03-04T10:15:30 (with the zone when it is known), RFC 3339 2024-03-04T10:15:30+05:30.
• Numeric: 04/03/2024, 03/04/2024, 04.03.2024, 04-03-2024, 2024/03/04.
• With month names: 4 March 2024, March 4, 2024, 04-Mar-2024, 04 Mar 2024 — in English or another language.
• Numbers: Unix seconds and milliseconds, Excel serials in the 1900 and 1904 systems.
• Your own pattern from YYYY, YY, MMMM, MMM, MM, M, DD, D, dddd, ddd, HH, H, hh, h, mm, ss, SSS, A, Z and [text in brackets].

Limitations

• Text files up to 100 MB and workbooks up to 50 MB, as far as your device’s memory allows; the page shows the first 200 rows, and the files have all of them.
• Plain numbers are read as dates only in columns named like dates (date, created, dob, created_at …): by their size, five digits as an Excel serial and ten or thirteen as a Unix timestamp, or as you choose under how numbers are read. Columns of amounts, years or counts are never turned into dates unless you tick them.
• Dates before the year 1 and calendars other than the Gregorian one are not read.
• Time zone abbreviations such as IST, EST or CST are not read, because each names more than one zone: choose the zone instead.

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