No as you dont change a global setting. Only the USER (deviceowner) can change the default date time formats. Most probably when setting the language.
The setting is just for your app. As long as the apps process is running the setting will be active.
You can set it back to the default values if you want...
If you want to set it back to the defaults use
DateTime.DateFormat = DateTime.DeviceDefaultDateFormat
DateTime.TimeFormat = DateTime.DeviceDefaultTimeFormat
But it is a setting only for your app while it runs. If you quit your app the setting will be the device-default for all other apps.