If dd.mm.yy is your system date format and you use date constant, not reference on the cell with the date, when ("4.3.19"+0) or --"4.3.19" could work (it converts text with the date representation into number). In general better to use the function, e.g. DATE().
If you use reference on the cell be sure you have real dates, not texts. If in A1 is 28.2.19 you may check if =A1+1 returns March 01.