Getting nil from dateFromString when converting string to date

Every time I try to pass a date string to the dateFromString function I get nil. I haven't read anywhere that things have changed since the iOS 7 update, but I am current on updates if that makes a difference on whether or not this still works the same way.

This is the code I'm using to create the date from string:

NSDateFormatter *dateFormat = [[NSDateFormatter alloc] init];
[dateFormat setTimeZone:[NSTimeZone systemTimeZone]];
[dateFormat setLocale:[NSLocale systemLocale]];
[dateFormat setDateFormat:@"yyyy-MM-dd hh:mm:ss"];
[dateFormat setFormatterBehavior:NSDateFormatterBehaviorDefault];

and my date string is @"2015-08-22 13:00:00"

NSDate *date = [[NSDate alloc] init];
date = [dateFormat dateFromString:@"2015-08-22 13:00:00"];
return date;

The last line returns nil.