Hi and welcome to the forum
If your 17.05.13 is text, which I suspect it is, and assuming it is in A1, this will adjust it to "the day before"...
=LEFT(A1,FIND(".",A1,1)-1)-1&MID(A1,FIND(".",A1,1),99)
Adjust references as needed
If it is in fact a date, the just subtract 1
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