objective-c计算相对于现在的时间差

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最近做了一个小应用程序，是读取新浪微博的。微博上面对于新发的微博，不是告诉你具体什么时候发布的，而是告诉你几秒钟之前，几分钟之前，几个小时之前之类的相对于现在的时间。可以使用下面代码来计算这个时间差。

- (NSString*)timestamp
{
// Calculate distance time string
//
time_t now;
time(&now);

int distance = (int)difftime(now, createdAt);
if (distance <0) distance =0;

if (distance <60) {
self.timestamp = [NSString stringWithFormat:@"%d %s", distance, (distance ==1) ?"second ago" : "seconds ago"];
}
elseif (distance <60*60) {
distance = distance /60;
self.timestamp = [NSString stringWithFormat:@"%d %s", distance, (distance ==1) ?"minute ago" : "minutes ago"];
}
elseif (distance <60*60*24) {
distance = distance /60/60;
self.timestamp = [NSString stringWithFormat:@"%d %s", distance, (distance ==1) ?"hour ago" : "hours ago"];
}
elseif (distance <60*60*24*7) {
distance = distance /60/60/24;
self.timestamp = [NSString stringWithFormat:@"%d %s", distance, (distance ==1) ?"day ago" : "days ago"];
}
elseif (distance <60*60*24*7*4) {
distance = distance /60/60/24/7;
self.timestamp = [NSString stringWithFormat:@"%d %s", distance, (distance ==1) ?"week ago" : "weeks ago"];
}
else {
static NSDateFormatter *dateFormatter = nil;
if (dateFormatter == nil) {
dateFormatter = [[NSDateFormatter alloc] init];
[dateFormatter setDateStyle:NSDateFormatterShortStyle];
[dateFormatter setTimeStyle:NSDateFormatterShortStyle];
}

NSDate *date = [NSDate dateWithTimeIntervalSince1970:createdAt];
self.timestamp = [dateFormatter stringFromDate:date];
}
return timestamp;
}

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