System Settings/Formats
Not all countries and languages are using the same format to display numbers, time, currency and other units.
For example in France, you would write a date in this "dd/mm/yyyy" format and in the US, you would instead use the "yyyy.mm.dd" format.
Plasma let you configure exactly how you want to read these formats.
Note
You can look at the description section, to see a live preview of your change.