Formatting dates retrieved from an API (ugh!) Can anyone do a Rick Flair on my NSDateFormatter?

I am pulling down a date-time string from an API, and preparing it for display on a subclass of UITableViewController.

I need to display "XX minutes ago", referring to a weather observation that has taken place in the past, usually not more than 60 minutes ago.

I am having trouble, like all other newbies, figuring out how to format the various NSDate objects. My code is below. To explain...

I've created a weatherDictionary object using [NSJSONSerialization JSONObjectWithData].

Line 1 is where I pull out the observationTime string from the weatherDictionary.

From the APO, the observationTime string is formatted as such:

"Tue, 08 Jul 2014 16:30:13 -0400",

Note the format of the hours:minutes:seconds

Line 2 is where I separate out the key component of that text string by splitting the string into an NSArray.

Line 3,4 is the dateFormatter. Line 5,6 is now, and the time of observation. The objectAtIndex:4 is the hours:minutes:seconds object of the array.

The line that I'm sure is causing the trouble is where I format the string *timeAgo.

NSString *observationTime = [[weatherDictionary valueForKey:@"current_observation"] valueForKey:@"observation_time_rfc822"];
    NSArray *observationTimeArray = [observationTime componentsSeparatedByString:@" "];
    NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
    [dateFormatter setDateFormat:@"HH:mm:ss"];
    NSDate *dateNow = [NSDate date];
    NSDate *observationDate = [dateFormatter dateFromString:[observationTimeArray objectAtIndex:4]];

    NSString *timeAgo = [NSString stringWithFormat:@"%f minutes ago", [dateNow timeIntervalSinceDate:observationDate]];

    self.timeSinceObservation = timeAgo;

When I log this to the console I get:

458081411.768475 minutes ago

NSDate Woooooooo!!