Unexpected result when using String.valueOf() to get the value of a Date and DateTime, when using an Object reference.

BehaviorUnexpectedForStringValueOfWithDateAndDateTime.cls

// Asserts fail for the following
/////////////////////////////////

DateTime dt = DateTime.now(); // VARIABLE_ASSIGNMENT dt = 2013-10-03T05:35:37.373Z
System.assertEquals(String.valueOf(dt), String.valueOf((Object) dt));
// Assertion Failed: Expected: 2013-10-02 22:35:37, Actual: 2013-10-03 05:35:37

Date d = Date.today(); // VARIABLE_ASSIGNMENT d = 2013-10-02T00:00:00.000Z
System.assertEquals(String.valueOf(d), String.valueOf((Object) d));
// Assertion Failed: Expected: 2013-10-02, Actual: 2013-10-02 00:00:00



// All below assertions pass as expected
////////////////////////////////////////

Blob b = Blob.valueOf('foobar');
System.assertEquals(String.valueOf(b), String.valueOf((Object) b));

Boolean bo = true;
System.assertEquals(String.valueOf(bo), String.valueOf((Object) bo));

Double db = 1.2345;
System.assertEquals(String.valueOf(db), String.valueOf((Object) db));

Decimal dc = 678.91011;
System.assertEquals(String.valueOf(dc), String.valueOf((Object) dc));

Integer i = 42;
System.assertEquals(String.valueOf(i), String.valueOf((Object) i));

Long l = 2147483648L;
System.assertEquals(String.valueOf(l), String.valueOf((Object) l));

String s = 'foobar';
System.assertEquals(String.valueOf(s), String.valueOf((Object) s));

Id id = '001000000000000AAA';
System.assertEquals(String.valueOf(id), String.valueOf((Object) id));

Time t = DateTime.now().time();
System.assertEquals(String.valueOf(t), String.valueOf((Object) t));

Object o = null;
System.assertEquals(String.valueOf(o), String.valueOf(o));

class FooBar {
    Integer ii = 1;
    String ss = 'baz';
}


FooBar f = new FooBar();
System.assertEquals(String.valueOf(f), String.valueOf((Object) f));

class FooBarWithToString {
    Integer ii = 1;
    String ss = 'baz';
    
    public override String toString() {
        return ii + '-' + ss;
    }
}

FooBarWithToString ff = new FooBarWithToString();
System.assertEquals(String.valueOf(ff), String.valueOf((Object) ff));
System.assertEquals(ff.toString(), String.valueOf(ff));
System.assertEquals(ff.toString(), String.valueOf((Object) ff));

Strings.cls

// Workaround wraps the String.valueOf() and casts back to Date/DateTime
// object if Object o is an instance of a Date/DateTime, otherwise the Object is
// simply passed straight through to String.valueOf()
public class Strings {
    public static String valueOf(Object o) {
        if (o instanceof Date) {
            return String.valueOf((Date) o);
        }

        if (o instanceof DateTime) {
            return String.valueOf((DateTime) o);
        }

        return String.valueOf(o);
    }
}

StringsTest.cls

@isTest
public StringsTest {
    @isTest static void testValueOfWithDateTime() {
        DateTime dt = DateTime.now();
        System.assertEquals(String.valueOf(dt), Strings.valueOf((Object) dt));
    }

    @isTest static void testValueOfWithDate() {
        Date d = Date.today();
        System.assertEquals(String.valueOf(d), Strings.valueOf((Object) d));
    }
}