That is because :
1. Your cells are formatted as dates
2. You entered a date as a continuous string "300623" which is the background value of the date you get ( or the 300623th day after Jan 1st 1900 which is day 1)

You have to enter the date as you would like to see it: first the day, then / then the month then / then the year
Formatting is how XL shows things to the outside world,the underlying value, in this case is still a number
If you wish to enter a continuous string and that it appears as a date, some VBA or Power Query will help

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