ちなみに10.12で確認したところ-[NSDateFormatter stringFromDate:]の場合こんな感じでICUを呼んでる
* frame #0: 0x00007fffdf0753cd libicucore.A.dylib` udat_open
frame #1: 0x00007fffca79f851 CoreFoundation` __cficu_udat_open + 65
frame #2: 0x00007fffca79eaf9 CoreFoundation` __ResetUDateFormat + 425
frame #3: 0x00007fffca81ef90 CoreFoundation` __SetUpCFDateFormatter + 480
frame #4: 0x00007fffcc184047 Foundation` -[NSDateFormatter _regenerateFormatter] + 329
frame #5: 0x00007fffcc183d90 Foundation` -[NSDateFormatter stringForObjectValue:] + 303