There are few ways to resolve that, the most common and simple one would be to format the cells prior pasting the data as text (select range, right click, format cells, double click on text).

What I have done was to simply change the decimal separator:

File -> options -> advanced -> untick “use system separators” and set the decimal separator to a dot “.” (default in my case was the comma).

File ->

Select File

Options ->

excel fix converting values to dates 2

Advanced -> untick “use system separators” and set the decimal separator to a dot “.” 

excel fix converting values to dates 3