Hi (java) people. Hope this simple logic will be useful.

static String solve( int year ) {

    if( year == 1918 ) return "26.09.1918";
    if( isLeap( year ) ) return "12.09." + Integer.toString( year );
    else return "13.09." + Integer.toString( year );

}

static boolean isLeap( int year ) {

    if( year % 4 != 0 ) return false;
    if( year > 1918 && year % 100 == 0 && year % 400 != 0 ) return false;
    return true;

}