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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchExcel stores dates and times as numbers, not as special date or time objects. The whole-number portion counts days, the decimal portion represents part of a day, and the cell’s number format controls how that value appears. A workbook’s 1900 or 1904 date system also affects how a serial number maps to a calendar date—which is why dates can shift when copied between workbooks.
What Excel stores in a date or time cell
Excel represents dates with sequential serial numbers so it can perform calculations on them. In the 1900 date system, January 1, 1900 is serial 1. For example, Microsoft documents January 1, 2025 as serial 45658 in that system: it is 45,657 days after January 1, 1900. Microsoft’s date-system and format documentation explains the serial model.
The integer portion identifies the day; the fractional portion represents the time within that day. A value of 0.5 means half a day has elapsed, or noon. Microsoft explains that times are stored as decimal fractions because time is treated as a portion of a day. Microsoft’s NOW function documentation gives the serial-number example and notes that NOW returns a serial date/time value.
Why the displayed date can differ from the stored value
A cell’s appearance is determined by its number format, not by a separate date value. Apply General to a numeric date or time cell and Excel can show the serial number and decimal fraction; apply a date or time format and Excel displays a calendar date or clock time instead. The underlying number remains available for arithmetic. See Microsoft’s guide to formatting numbers as dates or times.
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Excel’s format codes control which parts appear. Common date codes include d or dd for day, mmm for an abbreviated month, and yyyy for a four-digit year. Time codes include h:mm, h:mm:ss, and AM/PM. In a combined date/time format, m or mm next to an hour code or immediately before seconds means minutes; elsewhere it can mean month. Use [h]:mm to show elapsed hours above 24 rather than restarting at zero after a day.
- If a date appears as a serial number, apply a date format.
- If a date or time displays as
#####, the column may be too narrow to show it. - Typed values such as
2/2can be interpreted and displayed according to regional settings. If the entry must remain literal text, set the cell to Text or otherwise enter it deliberately as text.
Why dates shift between workbooks: the 1900 and 1904 systems
Excel workbooks can use either the 1900 or 1904 date system. The same calendar date has serial numbers that differ by 1,462 days between them—four years and one day, including a leap day. Microsoft’s example for July 5, 2011 is 40729 in the 1900 system and 39267 in the 1904 system. Microsoft’s explanation of Excel date systems describes the offset and copying behavior.
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When a numeric value is copied from a workbook using one system into a workbook using the other, the receiving workbook may interpret that serial according to its own date system, producing a date shifted by that offset. Excel documents automatic conversion options for copying between workbooks; chart dates copied from a 1904-system workbook may require manual correction.
Check the workbook rather than assuming the system from the computer’s operating system. Microsoft describes the Windows default as 1900 and the Mac default as 1904 on its date-system page, while its other documentation describes newer Excel versions as using 1900. Defaults and platform descriptions can vary across versions and documentation, so the workbook setting is the reliable check.
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Check the workbook date system
Microsoft’s documented Windows path is File > Options > Advanced, then the “Use 1904 date system” option. Microsoft’s Mac instructions place the setting under Excel Preferences and calculation preferences. These are documented paths, not guaranteed labels for every Excel version. Before changing a workbook setting, consider whether existing dates and formulas depend on the current system.
Convert text that looks like a date
A cell may contain text such as 2025-01-01 rather than a numeric date serial. Formatting text as a date does not by itself convert the text into a number. DATEVALUE converts text that Excel recognizes as a date into a serial value, but what Excel recognizes depends on date conventions and system context. If the text omits a year, Microsoft says Excel uses the computer’s current year; time information in the text is ignored. See Microsoft’s DATEVALUE reference and Microsoft’s guidance on converting text dates.
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- Keep a copy of the original text values so the source can be restored or checked.
- Identify the date order and locale used in the source—for example, whether the first number means month or day.
- Use four-digit years where possible, then convert using DATEVALUE or an appropriate Excel conversion method.
- Format the converted values as dates and validate sample rows against the original data before replacing it.
For dates constructed from separate year, month, and day values, DATE(year,month,day) returns a serial number. Use a four-digit year to avoid two-digit-year ambiguity, then apply a date format to display it as a date. DATE can normalize out-of-range month or day inputs—for example, a day beyond the end of a month may roll into the next month—instead of rejecting every invalid-looking input. Microsoft’s DATE function documentation describes its arguments and behavior.
Use date serials for calculations
Because numeric dates count days, subtracting a start date from an end date gives the number of days between them. Microsoft’s DAYS function documentation describes that calculation when both arguments are numeric dates. Time fractions work the same way: subtracting two date/time serials gives a duration measured in fractions of a day.
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NOW() returns the current date and time as a serial value. Microsoft’s examples use NOW()-0.5 for twelve hours earlier and NOW()+7 for seven days later. NOW updates when the worksheet recalculates or a macro runs; it does not update continuously. Applying an appropriate date/time format makes the result readable.
A practical way to diagnose a date problem
When a date looks wrong, work through these checks in order; they separate display issues from conversion and date-system problems.
Quick Recap
- Inspect the format: change the cell to General to see whether it contains a numeric serial or remains text.
- Check the workbook system: verify whether each workbook uses 1900 or 1904, especially after copying values between files.
- Check the source text’s date order: establish the locale and whether dates are month-first or day-first before converting.
- Test the conversion: use a four-digit year when constructing dates, format the result, and compare a few converted rows with the source.
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