NSDate category to deal with NTP timestamps

gistfile1.m

// yoinked and recrafted from Gavin Eadie's ios-ntp http://code.google.com/p/ios-ntp/
// NB - not perfectly symmetrical, this evaluates to NO:
//  NSDate* now = [NSDate date]; [now isEqualToDate:[NSDate dateWithNTPTimestamp:[now NTPTimestamp]];

struct NTPTimestamp {
    uint32_t seconds;
    uint32_t fractionalSeconds;
};
typedef struct NTPTimestamp NTPTimestamp;

static inline NTPTimestamp NTPTimestampMake(uint32_t seconds, uint32_t fractionalSeconds) {
    return (NTPTimestamp){seconds, fractionalSeconds};
}

static NSTimeInterval NTPTimestampDifference(NTPTimestamp start, NTPTimestamp end) {
    int a;
    unsigned int b;
    a = end.seconds - start.seconds;
    if (end.fractionalSeconds >= start.fractionalSeconds) {
        b = end.fractionalSeconds - start.fractionalSeconds;
    } else {
        b = start.fractionalSeconds - end.fractionalSeconds;
        b = ~b;
        a -= 1;
    }

    return a + b / 4294967296.0; // 2^32
}

// 1970 - 1900 in seconds 2,208,988,800 | First day UNIX
// 1 Jan 1972 : 2,272,060,800 | First day UTC
#define JAN_1970 0x83aa7e80
static NTPTimestamp NTPTimestamp1970 = {JAN_1970, 0};    // network time for 1 January 1970, GMT


@interface NSDate (PEAdditions)
+ (instancetype)dateWithNTPTimestamp:(NTPTimestamp)timestamp;
- (NTPTimestamp)NTPTimestamp;
@end

@implementation NSDate (PEAdditions)
+ (instancetype)dateWithNTPTimestamp:(NTPTimestamp)timestamp {
    return [NSDate dateWithTimeIntervalSince1970:NTPTimestampDifference(NTPTimestamp1970, timestamp)];
}
- (NTPTimestamp)NTPTimestamp {
    double integerValue;
    double fractionalValue = modf([self timeIntervalSince1970], &integerValue);
    fractionalValue *= 4294967296.0;
    return NTPTimestampMake(JAN_1970 + integerValue, fractionalValue);
}
@end