XML Schema: element attribute as unique value?

I'm making XSD schema to validate my XML file generated from java. Its about weather and I want to make value inside date unique.

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified" attributeFormDefault="unqualified">
<xs:element name="forecast">
<xs:complexType>
<xs:sequence>
<xs:element name="city" maxOccurs="unbounded">
<xs:complexType>
<xs:sequence>
<xs:element name="weather">
<xs:complexType>
<xs:sequence>
    <xs:element name="date" maxOccurs="unbounded">
        <xs:complexType>
            <xs:sequence>
                <xs:element name="maxtemp">
                    <xs:simpleType>
                        <xs:restriction base="xs:int"/>
                    </xs:simpleType>
                </xs:element>
                <xs:element name="mintemp" type="xs:int"/>
                <xs:element name="cloudcover">
                    <xs:simpleType>
                        <xs:restriction base="xs:int">
                            <xs:minInclusive value="0"/>
                            <xs:maxInclusive value="100"/>
                        </xs:restriction>
                    </xs:simpleType>
                </xs:element>
                <xs:element name="humidity">
                    <xs:simpleType>
                        <xs:restriction base="xs:int">
                            <xs:minInclusive value="0"/>
                            <xs:maxInclusive value="100"/>
                        </xs:restriction>
                    </xs:simpleType>
                </xs:element>
                <xs:element name="precipitation" type="xs:double"/>
                <xs:element name="windspeed">
                    <xs:simpleType>
                        <xs:restriction base="xs:int">
                            <xs:minInclusive value="0"/>
                            <xs:maxInclusive value="100"/>
                        </xs:restriction>
                    </xs:simpleType>
                </xs:element>
                <xs:element name="description">
                    <xs:simpleType>
                        <xs:restriction base="xs:string">
                            <xs:enumeration value="Heavy rain"/>
                        </xs:restriction>
                    </xs:simpleType>
                </xs:element>
            </xs:sequence>
            <xs:attribute name="value" use="required">
                <xs:simpleType>
                    <xs:restriction base="xs:string">
                        <xs:pattern value="\d\d-\d\d-\d\d"/>
                    </xs:restriction>
                </xs:simpleType>
            </xs:attribute>
        </xs:complexType>
        <xs:unique name="dateuq">
            <xs:selector xpath="date"/>
            <xs:field xpath="@value"/>
        </xs:unique>
    </xs:element>
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:sequence>
<xs:attribute name="name" type="xs:string"/>
<xs:attribute name="zip" use="required">
<xs:simpleType>
<xs:restriction base="xs:positiveInteger">
<xs:totalDigits value="5"/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
</xs:complexType>
</xs:element>
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:schema>

And here is test XML file:

<forecast> 
    <city name="Podgorica" zip="81000">
        <weather>
            <date value="01-01-14">
                <maxtemp>10</maxtemp>
                <mintemp>5</mintemp>
                <cloudcover>40</cloudcover>
                <humidity>20</humidity>
                <precipitation>10.2</precipitation>
                <windspeed>3</windspeed>
                <description>Heavy rain</description>
            </date>
            <date value="01-01-14">
                <maxtemp>10</maxtemp>
                <mintemp>5</mintemp>
                <cloudcover>40</cloudcover>
                <humidity>20</humidity>
                <precipitation>10.2</precipitation>
                <windspeed>3</windspeed>
                <description>Heavy rain</description>
            </date>
        </weather>
    </city>
</forecast>

So this should not be passing validation here because value should be unique, why this isn't working?

You placed the uniqueness constraint in date, but it doesn't make any sense in that context. You can declare an uniqueness constraint for date in the weather context (or in a higher context such as city or forecast.

To guarantee an unique data for each weather declare it inside the weather element declaration:

<xs:element name="weather">
    <xs:complexType>
        <xs:sequence>
            <xs:element name="date" maxOccurs="unbounded">
                <xs:complexType> 
                 ...
                </xs:complexType>
                <!-- remove it from here -->
            </xs:element>
        </xs:sequence>
    </xs:complexType>
    <!-- and place it here -->
    <xs:unique name="dateuq">
        <xs:selector xpath="date"/>
        <xs:field xpath="@value"/>
    </xs:unique>
</xs:element>