SOAP API update DateTime Field
<?xml version="1.0" encoding="UTF-8"?>
<SOAP-ENV:Envelope xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:ns1="marketo.com/mktows/"><SOAP-ENV:Header><ns1:AuthenticationHeader><mktowsUserId>completesolar1_70493473515C4EE90C2019</mktowsUserId><requestSignature>cfcbe2135f3c49caf96e8af3dccc65b4031256ae</requestSignature><requestTimestamp>2014-08-06T14:25:53-04:00</requestTimestamp></ns1:AuthenticationHeader></SOAP-ENV:Header><SOAP-ENV:Body><ns1:paramsSyncMultipleLeads><leadRecordList><leadRecord><Id>1060045</Id><Email>c926f676-1f57-3f9f-fa83-52a8eaa340d1@none.com</Email><ForeignSysPersonId xsi:nil="true"/><ForeignSysType xsi:nil="true"/><leadAttributeList><attribute><attrName>next_meeting_c</attrName><attrType></attrType><attrValue></attrValue></attribute></leadAttributeList></leadRecord></leadRecordList><dedupEnabled xsi:nil="true"/></ns1:paramsSyncMultipleLeads></SOAP-ENV:Body></SOAP-ENV:Envelope>
What does the attrValue need to be set to blank out the data?