I would like to inform that when users launch their streaming sessions, they can select Regional settings from the gear menu on the AppStream 2.0 toolbar to set any of the Time zone, Locale, and Input method options. Any changes a user makes will apply instantly to the user's streaming session as well as to any future sessions launched by the user in the same AWS Region. Also, if a user previously selected a user locale or input method when streaming from any fleet instance in the same Region, those user-specified settings automatically override any default user locale and input method that you specify through your image builder.

So if you have any custom settings defined in the image or user has defined some date & time formatting at the OS level, those settings will get overridden using the regional settings option. Even if application settings persistence is enabled at the stack level, date and time format will still get reverted by the regional settings defined by the user.

Reference

Configure Default Regional Settings for Your AppStream 2.0 Users https://docs.aws.amazon.com/appstream2/latest/developerguide/configure-default-regional-settings.html#configure-default-time-zone-2016-2019

Enable Your AppStream 2.0 Users to Configure Their Regional Settings https://docs.aws.amazon.com/appstream2/latest/developerguide/regional-settings.html