You copy a schedule into another workbook and the dates move by about four years. Changing the display from a short date to a long date will not repair the underlying value. Excel's 1900 and 1904 date systems are worth checking first.
Compare the settings before correcting cells
Current Excel versions normally use the 1900 system, including modern Excel for Mac. Older Mac workbooks may use 1904; the computer's brand alone does not tell you what a particular file uses.
In desktop Excel for Windows, inspect File → Options → Advanced, under the calculation settings for that workbook. On Mac, look in Excel's preferences under Calculation. Compare the option for the 1904 date system in both files without changing it yet.
As a worked example, 29 September 2026 has serial value 46294 in the 1900 system and 44832 in the 1904 system. The difference is 1462 days. That pattern is a clue, not an instruction to add 1462 to an entire column: some rows may already have been converted or manually repaired.
Verify a copy through the whole workflow
Keep the originals. Compare dates with an independent schedule and check whether the imported cells contain date values or text. Use the conversion offered by your Excel version where appropriate, then check records from different months and years.
Inspect formulas and chart dates separately. Microsoft warns that dates copied as part of a chart are not automatically converted from the 1904 system. Save the corrected copy under a new name and note which data was converted. Merely toggling a workbook setting can affect other dates too.