This Gist wll get the releasedate for the version in the fixversions field in Jira and returns dueDate if not available.

getReleaseDateofVersion.sil

//Gather due date based on fixed version or issue due date

string getvers;

if (isNull(fixVersions)) {
  return dueDate;
  }
else {
    getvers = fixVersions;
    return admGetProjectVersion(project, getvers).releaseDate;
    //return admGetProjectVersions(project);
    //return fixVersions;
}