Hi,
in Arnolds exr files we have an rendertime attribute what it quite helpful. In ext files created from mantra renderings I could not find such an attribute. What I found is ctime, mtime and capDate. In theory I'd say the mtime-ctime is the rendertime, but this seems not to be the case if an image is rerendered (and overwritten) because the ctime may be from another renderjob. So I found the capDate what seems to be similiar to the render start time, can this value be used to calculate the rendertime?