User Configuration > Administrative Templates > Microsoft Excel 2016 > Miscellaneous
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HKCU\software\policies\microsoft\office\16.0\excel\options
When this setting is not enabled, Excel follows the Short date style setting under Regional Settings in Control Panel. When this setting is enabled, Excel always displays four digits when you type a date that includes a four-digit year, which may override the Short date style setting under Regional Settings in Control Panel.