DateTime endTime = sfdc.getServerTimestamp().timestamp;

DateTime startTime = endTime.Subtract(new System.TimeSpan(1, 0, 0, 0, 0));

sforce.QueryResult qr = sfdc.query("Select Id from Asset Where SystemModstamp > " + startTime);

The error is:

System.Web.Services.Protocols.SoapException was unhandled
  Message="MALFORMED_QUERY: \nAsset Where SystemModstamp > 11/27/2006 8:59:45 PM\n                            

   ^\nERROR at Row:1:Column:47\nunexpected char: '/'"