You can use Patterns for example :

String str = "Niedersachsen,NOT IN CHARGE SINCE: 03.2009, CATEGORY";
Pattern p = Pattern.compile("\\d{2}\\.\\d{4}");
Matcher m = p.matcher(str);

if (m.find()) {
    System.out.println(m.group());
}

Output

03.2009

Note : if you want to get similar dates in all your String you can use while instead of if.

Edit

Or you can use :

String str = "Niedersachsen,NOT IN CHARGE SINCE: 03.03.2009, CATEGORY";
Pattern p = Pattern.compile("SINCE:(.*?)\\,");
Matcher m = p.matcher(str);

if (m.find()) {
    System.out.println(m.group(1).trim());
}

use string methods

for getting a part of a string, use the method

<your string>.substring(<start index>, <end index>)

additionally, you can use the method

<your string>.indexOf(<character>)
in order to get the index of a given character. this can help you find index values for your string if you don't know them or want to use a character as the reference instead.

The accepted answer is correct but it doesn't tell you how to use it. This is how you use indexOf and substring functions together.

String filename = "abc.def.ghi";     // full file name
int iend = filename.indexOf("."); //this finds the first occurrence of "." 
//in string thus giving you the index of where it is in the string

// Now iend can be -1, if lets say the string had no "." at all in it i.e. no "." is found. 
//So check and account for it.

String subString;
if (iend != -1) 
{
    subString= filename.substring(0 , iend); //this will give abc
}