The input string for dates in Google Docs's spreadsheet app seems to be set to en_US, which is quite unnatural to me.
How can I change the input format?
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Select the locale with the date format you want (for example US is
mm/dd/yyyy and UK
The answer from @Metalshark is great for updating existing documents, but I'd also like to point out that this can be set as a default.
Click the gear icon on the right from your main Google Drive dashboard and select Settings. If you don't have a language selected you can set it here. This should apply to all future documents.