oiio writer: update the DateTime metadata format

* Tuttle should continue to put it since all oiio plugins do not add it.
* Update the date format to write the same as oiio.
* See an example with TIFF:
https://github.com/OpenImageIO/oiio/blob/Release-1.6.16/src/tiff.imageio/tiffoutput.cpp#L475

plugins/image/io/OpenImageIO/src/writer/OpenImageIOWriterProcess.tcc

@@ -548,7 +548,7 @@ void OpenImageIOWriterProcess<View>::writeImage(View& src, const std::string& fi
     const time_t rawtime = time(0);
     const struct tm * timeinfo = localtime(&rawtime);
     char buffer[80];
-    strftime(buffer, 80,"%d-%m-%Y %I:%M:%S", timeinfo);
+    strftime(buffer, 80,"%Y:%m:%d %I:%M:%S", timeinfo);
     spec.attribute("DateTime", std::string(buffer));
 
     if(! params._project.empty())