Q:

Colums validation gives error

I want to create an input form in SharePoint. 
In this form you have to select a date. It must be the date 2016/01/20 or 2016/01/27 (in my country it's called 20.01.2016 and 27.01.2016. The column is called Datum). When you try another date, an error message, which I define should appear.
I tried this code: 
=OR(Datum="20.01.2016";Datum="27.01.2016")

The Error message, which I defined, appears. But the problem is, it appears all the time. Even if I enter the correct date. 
What's wrong with this code?

A:

I tried it in my German SharePoint 2013 on-prem environment, but maybe it is working in SPO as well.
The formular itself after you saved it looks like this:

=ODER(Datum=DATWERT("20.01.2016");Datum=DATWERT("27.01.2016"))
You can also use the english formular:
=OR(Datum=DATEVALUE("20.01.2016");Datum=DATEVALUE("27.01.2016"))
Assuming your "Datum" field is a Date type, then you need to compare dates to dates and not dates to strings. I think that's the reason.
The english formular will be translated into german after you saved it.

