ios - NSDateFormatter 毫秒错误

Question

我想创建一个 NSDateFormatter 来解析这样的日期“2014-05-13 23:31:41.374577”。所以:

NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
formatter.dateFormat = @"yyyy-MM-dd HH:mm:ss.SSSSSS";
NSString *dateString = @"2014-05-13 23:31:41.374577";
NSDate *date = [formatter dateFromString:dateString];
NSString *anotherDateString = [formatter stringFromDate:date];

但是，anotherDateString 是 2014-05-13 23:31:41.374000。为什么它会截断毫秒？

Answer 1

NSDateFormatter 似乎只适用于毫秒分辨率，因为以下原因:

• 通过在 CFDateFormatterCreateDateFromString 中设置断点，可以看到这个函数是从 dateFromString::

  调用的

  (lldb) bt
  * thread #1: tid = 0x26d03f, 0x018f47d0 CoreFoundation`CFDateFormatterCreateDateFromString, queue = 'com.apple.main-thread', stop reason = breakpoint 3.1
      frame #0: 0x018f47d0 CoreFoundation`CFDateFormatterCreateDateFromString
      frame #1: 0x0116e0ea Foundation`getObjectValue + 248
      frame #2: 0x0116dfc7 Foundation`-[NSDateFormatter getObjectValue:forString:errorDescription:] + 206
      frame #3: 0x0116879f Foundation`-[NSDateFormatter dateFromString:] + 71
    * frame #4: 0x00002d56 foo`main(argc=1, argv=0xbfffee54) + 182 at main.mm:25
  

• CFDateFormatterCreateDateFromString() 来自 CFDateFormatter.c这是开源的。人们可以看到所有的日历计算都是使用 ICU Calendar Classes .

• 在calendar.h 中说明Calendar 使用具有毫秒分辨率的 UDate:

  /**
   * <code>Calendar</code> is an abstract base class for converting between
   * a <code>UDate</code> object and a set of integer fields such as
   * <code>YEAR</code>, <code>MONTH</code>, <code>DAY</code>, <code>HOUR</code>,
   * and so on. (A <code>UDate</code> object represents a specific instant in
   * time with millisecond precision. See UDate
   * for information about the <code>UDate</code> class.)
   * ...