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Sorry - just back from vacation.
A string is text - entering 23.10.18 into a cell will result in Excel treating it as text, since that is not a standard date format.
If you enter the number 43332 into a cell formatted as general, and change the number of decimals, the display should change from 43332 to 43332.0 43332.00 etc.
But, if you apply a date format to the same cell, the date 8/20/2018 (or some variant of that) will be displayed. As you change the date format, the display will change.
If you enter the string 20.08.18 into a cell, and change the format, the display will not change, since that is a string and not a number. If you enter 8/20/18 into a cell, Excel converts that to the number 43332 (internally), which is the number of days since 12/31/1899.
Any string you enter that has a number at the end - for example, Bernie123 - will have the final number incremented with the cell is drag-filled to other cells Bernie124, Bernie 125, etc. That is why the 18 is changing to 19, 20, etc.
So, enter a valid date and you should be able to increment it by dragging...