Right. I was thinking of something along the lines of:

-(NSString *)dateStringInUTCWithExtension:(NSString *) ext {
  return [self callDryHelper: [NSTimeZone timeZoneWithName:@"UTC"]];
}

to remove the duplication and avoid having to remember the format string etc where used. Very minor improvement. (Parkinson's Law of Triviality in action? :-)

Timezone offset in the file name might be nice, if easy to do.