Where, specifically, in Outlook?
Given that so many Windows programs use whatever format you've set up for your Region in Settings, Date & Time, Region Pane, using the Change data formats link, you may want to look at tweaking the "short date" format to look as you prefer to see if that does it. If not, if I know where in Outlook, I can see if it has its own setting or if one can choose to use the "long date" format rather than "short date" in that context.
Brian - Windows 10 Pro, 64-Bit, Version 20H2, Build 19042
To think is to differ.