Answer
This is a known Microsoft Excel behavior, not an IFS-side bug — it comes from how Excel interprets two-digit year numbers under certain regional/locale settings, and Microsoft's own documentation covers this ambiguity. The practical workaround on the IFS side is to have dates entered/exported using a 4-digit year format rather than a 2-digit one — this option exists in the client and avoids the century-interpretation ambiguity that triggers the problem in Excel.