How to get Effective Date and Expiration Date of ContractDocument

Hello Guys,

I'm facing diffculties to get  Effective Date and Expiration Date of ContractDocument in SAP CLM via Script. In ContractDocumentIBeanImpl' not have  a getEffective and Expiration date. I tried get for getExtensionField("EXPIRATION_DATE_DATE"), but I did not succeed and return Null with completed  field. I would like know if possible to get this fields via Script for manipulation ?