ONVIF GetSystemDateAndTime Request

You need to send there just a HTTP request as SOAP works over HTTP. For example via CURL it would be like this:

curl 10.10.10.10:1234/onvif/device_service -d '<s:Envelope xmlns:s="http://www.w3.org/2003/05/soap-envelope"><s:Body xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"><GetSystemDateAndTime xmlns="http://www.onvif.org/ver10/device/wsdl"/></s:Body></s:Envelope>'

As a result you will get something like this with some headers:

<tds:GetSystemDateAndTimeResponse>
    <tds:SystemDateAndTime>
        <tt:DateTimeType>Manual</tt:DateTimeType>
        <tt:DaylightSavings>false</tt:DaylightSavings>
        <tt:TimeZone>
            <tt:TZ>MoroccoStandardTime0</tt:TZ>
        </tt:TimeZone>
        <tt:UTCDateTime>
            <tt:Time>
                <tt:Hour>10</tt:Hour>
                <tt:Minute>5</tt:Minute>
                <tt:Second>35</tt:Second>
            </tt:Time>
            <tt:Date>
                <tt:Year>2014</tt:Year>
                <tt:Month>3</tt:Month>
                <tt:Day>14</tt:Day>
            </tt:Date>
        </tt:UTCDateTime>
    </tds:SystemDateAndTime>
</tds:GetSystemDateAndTimeResponse>

And also dont forget that most of the actions requires authorization headers included in to the request.

AUTHENTICATION

In ONVIF Application Programmer's Guide on page 35 is described how auth is done. For example it looks like this:

<s:Header>
    <Security s:mustUnderstand="1" xmlns="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-secext-1.0.xsd">
        <UsernameToken>
            <Username>admin</Username>
            <Password Type="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-username-token-profile-1.0#PasswordDigest">MuMnyh3wTxGWOCc=</Password>
            <Nonce EncodingType="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-soap-message-security-1.0#Base64Binary">8Qqve9KCkNhQAAAAAAA==</Nonce>
            <Created xmlns="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-utility-1.0.xsd">2014-03-04T14:03:05.130Z</Created>
        </UsernameToken>
    </Security>
</s:Header>

@Kirix solution didn't work for my particular camera. However, after a huge amount of code study and trail/error.. I determined that the Trendnet camera I have insists on the header being perfect. I figured out a curl variation:

curl http://192.168.xxx.xxx/onvif/device_service --data '<s:Envelope xmlns:s="http://www.w3.org/2003/05/soap-envelope"><s:Body xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"><GetSystemDateAndTime xmlns="http://www.onvif.org/ver10/device/wsdl"/></s:Body></s:Envelope>' --header 'Content-Type: application/soap+xml; charset=utf-8; action="http://www.onvif.org/ver10/device/wsdl/GetSystemDateAndTime";'