As of SP14 of SAP GRC NFE 1.0, this functionality is included in the standard solution.
For that, apply SAP Note 1451768 on GRC NFE and SAP Note 1441403 on ERP.
Follow a code sample to read the authorization date and time directly from XML.
The routine start reading the DOCID from table /xnfe/xml using 44 access key, after than use this value the read XML from KPro.
Using string routines to identify the position of tag dhRecbto, read their content and split their value into two variables EV_AUTHDATE and EV_AUTHTIME.
Remark: There's a constant value on this code (gc_sdok-class) that can be found on the include /XNFE/DB_CONSTANTS, you can insert the include line on the TOP include of group function, or directly on the code.