Date and time
Convert one value per line. Choose the workbook’s date system; results include time and flag the fictitious leap day.
Your input is processed in this browser and is not sent to a server.
1900: 1 = 1900-01-01. 1904: 0 = 1904-01-01. Select the source workbook’s system.
One decimal serial or YYYY-MM-DD per line; optional T or space + HH:mm:ss.SSS. Use a decimal point and no surrounding spaces or timezone suffix.
Calendar fields have no timezone. Day 60 in 1900 mode is fictitious; day 0 is an explicit 1899-12-31 extension.
Nearest millisecond, half up; serial input ≤15 fractional digits, output ≤12. Copy values requires all rows to succeed.
1000 physical lines / 100000 UTF-16 code units / 200 per line. Empty lines remain errors.
Enter input or choose an example to begin.
An Excel serial date converter helps when an exported column contains numbers such as 40729.5 and the spreadsheet formatting is gone. Before interpreting the column, check which date system the source workbook uses.
A plausible calendar date is not proof of a correct conversion. The same number means different dates in the two systems, and its fractional part represents a time of day rather than decimal seconds.
The Excel serial date converter maps 40729 to 2011-07-05 in the 1900 system; that same date is 39267 in the 1904 system. A visible setting makes the assumption reviewable before an entire column is interpreted.
Serials 59, 60 and 61 represent February 28, a fictitious February 29, and March 1 of 1900. The middle value is preserved and flagged instead of being quietly moved to a real day. Its fractional time can also be converted back.
In Excel, inspect “Use 1904 date system” in the workbook calculation settings. Do not infer it from Windows or Mac: modern Mac workbooks can use 1900. Changing this tool’s selector reinterprets the input, without migrating the workbook.
Example follows your selected direction and system. Preview shows the replacement row counts before Apply. In 1900 mode it includes zero, the fictitious day, and a noon value; Undo restores both your previous input and report.
Automatic conversion waits 300 ms. Turn it off for Convert or Ctrl/Cmd + Enter. Copy values and Reverse require every row to succeed; Copy report and Download retain errors. Edits immediately disable stale exports.
Convert an Excel date serial to date or a date to serial number. A date without a time means midnight. Full output uses YYYY-MM-DDTHH:mm:ss.SSS, with three fractional second digits and no timezone suffix.
A fraction of 0.5 means noon, 0.25 means 06:00 and 0.75 means 18:00. Decimal serials are rounded to the nearest millisecond, with exact half-millisecond ties rounded up. The report marks values that needed rounding.
Each physical line keeps its input and line number, including empty and invalid lines. The result view starts with 50 rows and expands by 50; both report exports always contain the whole batch.
Copy values produces one date or serial per line when all rows are valid. The TXT report also carries the selected system, direction and warnings. Reversal previews a new input column before replacing anything.
Yes. The integer identifies the day and the fraction is a share of 24 hours. For example, 40729.5 is 2011-07-05 at noon in the 1900 system. Displaying only the date in a spreadsheet can hide a time that remains in the stored number.
The supported final calendar day is 9999-12-31. Its integer serial is 2958465 in the 1900 system and 2957003 in the 1904 system. This tool permits time through 23:59:59.999 and rejects rounding that crosses the upper boundary.
Check the source workbook’s date system. Older Mac files may use 1904; the same modern date has a serial 1462 smaller than in 1900. Choose the matching system rather than shifting dates based only on the computer brand.
A date is stored as a number; a General-formatted cell can reveal it. Apply a date/time cell format if you want a date display. For text concatenation, Excel’s TEXT function can format the value. This converter does not modify cell formatting.
Preserving the calendar date requires adjusting the serials when the workbook system changes. For dates supported by both systems, the 1900 serial is 1462 greater. Here, decode under the source system, reverse to date input, then choose the target system; inspect the report before pasting back.
Unexpected result?
Use the Excel serial date converter with one value whose date you already know. Confirm the system and time before converting a full column, then retain the report with the source workbook.