How to use Datetime in Dynamic SOQL Query in Salesforce ?

This error is because of wrong construction of Dynamic Query with Datetime. following code snippet will give idea on how to construct dynamic query for Datetime ?

1	//format the datetime to make it Dynamic Soql ready

2	String formatedDt = cutOffDateTime.format('yyyy-MM-dd'T'HH:mm:ss'Z'');

3	String sql = 'SELECT  a.Id  FROM  Agents_Answer__c a    WHERE  a.Agents_Test_Result__r.Agent_Name__r.IsActive__c = false AND  LastModifiedDate < '+ formatedDt ;

Where, “cutOffDateTime” is variable of datetime type.