Why do dates imported from two schedule files sort into the wrong order?

A date’s appearance does not guarantee that the spreadsheet is treating it as a date. In Excel, dates need to be stored as date/time values; an entry Excel cannot recognize as a date may be stored as text, which can lead to unexpected ordering. [S1]

Imports can also be interpreted differently. Excel documents that import and automatic-conversion settings affect whether date-like text is converted, so don’t assume both files were handled the same way. [S3]

For example, 03/04/2025 could mean March 4 or 3 April, depending on the convention used by the source. Don’t convert an ambiguous value based only on how it looks. Keep an untouched copy of each file and find out which convention each source intended before standardizing.

Start with the date column in each original file, before combining or resorting anything:

1. Compare the headers and a sample of entries from both files. Note whether they use patterns such as 2025-04-03 and 4/3/25, and whether the source documentation explains the order.
2. Check for values that are blank, include unexpected text, or use two-digit years. Note these as exceptions rather than guessing what they mean.
3. In Excel, left alignment can be a visual clue that a value is stored as text rather than a date, but it is not proof on its own. Excel also notes that some two-digit-year text dates may show an error indicator when Error Checking is enabled. [S2]
4. Confirm the import settings and locale used for each file if the sort results are unexpected. Excel identifies locale as one factor to check. [S1]

Record the findings per file. The goal is to establish what each entry means and whether Excel recognizes it—not merely to make the cells look alike.

First resolve the meaning of each source format using its documented convention or a reliable source-file reference. If a value is ambiguous and you cannot establish its meaning, flag it for clarification instead of choosing a month and day yourself.

Then, in a working copy, convert text dates using a method appropriate to the confirmed format. Excel documents DATEVALUE for converting most text dates into date serial values, but it does not provide a universal rule for interpreting ambiguous formats; verify that conversion succeeds and means the intended date. [S2] Apply a date display format afterward. Formatting changes how a value appears; it does not, by itself, resolve an incorrectly interpreted date.

Keep the original imported column until you have checked the converted results. For instance, if a source confirms DD/MM/YYYY, verify that 04/03/2025 becomes the intended 4 March 2025—not 3 April—before relying on it.

Use a copy of the combined schedule. Check several converted dates against their original entries, paying particular attention to ambiguous values and records from each file. Confirm that entries intended to be dates are recognized as date values, and that unresolved or invalid entries are identified rather than silently included.

Then sort the verified date field using Excel’s Oldest to Newest option and inspect the beginning, end, and a few transitions in the result. Excel recommends checking that the imported column contains real date values before using its chronological sort options. [S1] If the order is still surprising, stop and recheck the source conventions, locale, and import behavior rather than repeatedly sorting the original data.

This guidance is Excel-specific. It does not establish how another spreadsheet app or a schedule-import system interprets dates; use that application’s documentation and verify its results separately. [S1] [S3]