datetime value in webservice how to use and smart idea?

I have the below webservice: POST /users.asmx HTTP/1.1
Host: 192.168.0.103
Content-Type: application/soap+xml; charset=utf-8
Content-Length: length

<?xml version="1.0" encoding="utf-8"?>
<soap12:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:soap12="http://www.w3.org/2003/05/soap-envelope">
<soap12:Body>
<SaveUserDetails xmlns="http://tempuri.org/">
<APIKey>string</APIKey>
<email>string</email>
<firstletters>string</firstletters>
<firstname>string</firstname>
<birthdate>dateTime</birthdate>
...