Excel Date Converter
Convert spreadsheet serial dates and civil date-times with explicit date systems and row-by-row errors.
This tool runs entirely in your browser. Your data is never uploaded, never stored, and never leaves your device.
Convert pasted spreadsheet serials and civil date-times using an explicit 1900 or 1904 system, with fractional times and per-row errors.
How to use it
- 1Check the source workbook's date system, then choose it explicitly.
- 2Select serial-to-date or date-to-serial and time precision.
- 3Paste one value per line; review converted rows and fictitious-date errors.
- 4Download CSV or JSON with all rows and assumptions.
Example
- Input
- 40729.5; 1900 system
- Output
- 2011-07-05T12:00:00.000
No timezone is inferred. Serial 60 in this system is rejected.
What happens to your data
Values stay locally in browser memory, without uploads, stored history or inputs in the URL. JSON keeps exact strings; CSV applies a formula-leading-cell safeguard.
Last updated October 2026
A spreadsheet date can turn into an unexplained number when it is exported, pasted or read through an API. This Excel date converter translates those serial values into readable civil date-times, or converts a date-time back into a serial. Choose the workbook's 1900 or 1904 date system rather than letting an apparently plausible answer hide a four-year error.
Paste one value per line and review individual errors alongside valid answers. The converter treats Excel's fictitious serial-60 date explicitly, offers second or millisecond rounding, and keeps the workbook assumption in the downloadable record. It operates on pasted values, not on uploaded workbooks, and does not guess a timezone.
How it works
First check the source workbook's date-system setting. Modern Excel commonly uses the 1900 system, but a workbook may use 1904 instead. The same real date differs by 1,462 serial days. For example, July 5, 2011 is 40729 in the 1900 system and 39267 in the 1904 system. Choosing the correct system matters more than the number of decimal places shown.
In the 1900 system, serial 1 is January 1, 1900. Excel preserves a historical compatibility error that treats 1900 as a leap year. Serial 59 corresponds to February 28, serial 60 refers to the nonexistent February 29, and serial 61 corresponds to March 1. This converter refuses serial 60, including its fractional times, instead of silently turning it into a different real date. Serial zero and negative serials are outside this tool's 1900-system scope. In the 1904 system, serial zero is the real January 1, 1904.
The fractional part represents a portion of a 24-hour civil day: .5 is noon. The decimal characters are read as an integer ratio before rounding, avoiding a binary-floating-point interpretation of the entered decimal. Choose nearest whole second or nearest millisecond. Exact halfway cases increase the fractional-day time. A rounding carry moves into the next real calendar date; it does not create the fictitious February 29. If rounding would move past the supported last date, the row receives an error.
For the reverse direction, enter YYYY-MM-DD or YYYY-MM-DDTHH:MM:SS, optionally with one to three fractional-second digits. A space may replace T. A date alone means midnight. Hours must be 00–23 and minutes/seconds 00–59. Leap seconds, UTC-offset suffixes, localized date formats and ambiguous slashes are deliberately rejected. These are spreadsheet civil values: converting 40729.5 does not establish a UTC instant or account for daylight saving.
Real dates range from January 1, 1900 or January 1, 1904, according to the chosen system, through December 31, 9999. Input serials accept at most 16 whole digits and 12 fractional digits, but dates outside those bounds are rejected. Serial output is rounded to at most 12 decimal places; recurring fractions of a day cannot be printed with infinite precision. A date-time round trip is intended at the selected second or millisecond resolution, not as preservation of every originally entered serial digit.
The batch accepts 500 nonblank rows and 50,000 characters. Blank rows are ignored while original line numbers remain visible. Each invalid row has its own error and does not suppress valid rows. The interface previews 25 rows; CSV and JSON contain every processed row. CSV prefixes an apostrophe to formula-leading cells as a spreadsheet safeguard; JSON preserves the exact input and output strings without that prefix. Values remain in browser memory without uploads, persistent storage or URL parameters. Changing any input or setting removes stale results and export buttons.
Worked example: A serial date with noon preserved
- Input
- 40729.5; 1900 date system; millisecond precision
- Result
- 2011-07-05T12:00:00.000
The same real date-time in the 1904 system has serial 39267.5. Neither value contains a timezone.
Common use cases
- Recover readable dates from CSV exports that contain spreadsheet serial numbers.
- Diagnose a 1,462-day shift between workbooks using different date systems.
- Convert a known civil date-time back into the serial required by an import.
- Keep row errors and the chosen date system together in an export.
Frequently asked questions
What if rounding a serial near 60 would cross the fictitious date?＋
A 1900-system value on serial day 59 that rounds to the next midnight is rejected rather than silently jumping past serial 60. Use a finer precision when it resolves the value, or supply an explicit real date-time. Serial-day 60 itself is always rejected in that system.
Why is my converted Excel date roughly four years wrong?＋
Check the workbook's 1900/1904 setting. The same real date differs by 1,462 serial days. Do not automatically choose the answer that merely looks plausible; establish the source system.
What happens to Excel serial 60?＋
In the 1900 system it represents Excel's fictitious February 29, 1900. The row is rejected, including fractional times on that fictitious day. In the 1904 system, serial 60 is an ordinary valid date.
Does the tool convert a serial into my local timezone?＋
No. A spreadsheet date serial does not itself identify a timezone. The output is a civil date-time with no Z or offset. Supply the original timezone separately if you later need an instant.
Why can the serial output have rounded digits?＋
Fractions such as one second divided by 86,400 seconds often repeat in decimal. Serial output uses at most 12 decimal places. The selected date-time resolution is one second or one millisecond, and any input rounding is disclosed.
Can I upload an Excel workbook to detect its system?＋
No. This page converts pasted scalar values. Inspect the source workbook settings yourself, then select its date system here. It does not read formulas or formatting.
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