A block of dates that moves by exactly four years and one day after copying deserves a different investigation from dates with reversed days and months. Excel supports two date systems, 1900 and 1904, whose serial values for the same date differ by 1,462 days.

Keep both original workbooks. Identify a date whose intended calendar day is independently known, and compare that same record before and after the transfer. A similar-looking year difference alone is not enough to justify a bulk correction.

Inspect each workbook before changing either

In desktop Excel for Windows, open File, Options, Advanced. Under When calculating this workbook, select the relevant workbook and inspect Use 1904 date system. Record the setting in both source and destination. Do not assume every file made on a Mac uses the older system.

The 1900 serial for a date is 1,462 greater than its 1904 counterpart. Microsoft documents adding 1,462 to correct dates that need to move later, and subtracting it when they need to move earlier. First prove the required direction on one confirmed affected record in a separate working copy.

For example, a helper formula =A2+1462 moves a numeric date in A2 later; =A2-1462 moves it earlier. Give the helper result a date display and compare it with the independently known day. These are alternatives, not two consecutive steps.

Correct only the confirmed transfer problem

Distinguish affected copied values from dates already entered correctly in the destination. Changing the workbook-wide system just to fix one pasted block is not a controlled correction for a mixed sheet.

Check several records across the affected range and at least one untouched neighbouring record. Include any reporting deadline or period boundary that depends on those dates. If offsets vary, stop applying the single-offset explanation and investigate the original input.

Review copied charts separately: Microsoft notes that dates copied as part of a chart are not automatically converted from the 1904 system. Keep the accepted correction and its affected range documented, so a later editor does not apply the same adjustment again.

Sources: Microsoft date systems; Microsoft date-system correction.