时间跨度的智能格式化

@interface NSDate (PrettyDate)

- (NSString *)prettyDate;

@end

@implementation NSDate (PrettyDate)

- (NSString *)prettyDate
{
    NSString * prettyTimestamp;

    float delta = [self timeIntervalSinceNow] * -1;

    if (delta < 60) {
        prettyTimestamp = @\"just now\";
    } else if (delta < 120) {
        prettyTimestamp = @\"one minute ago\";
    } else if (delta < 3600) {
        prettyTimestamp = [NSString stringWithFormat:@\"%d minutes ago\", (int) floor(delta/60.0) ];
    } else if (delta < 7200) {
        prettyTimestamp = @\"one hour ago\";      
    } else if (delta < 86400) {
        prettyTimestamp = [NSString stringWithFormat:@\"%d hours ago\", (int) floor(delta/3600.0) ];
    } else if (delta < ( 86400 * 2 ) ) {
        prettyTimestamp = @\"one day ago\";       
    } else if (delta < ( 86400 * 7 ) ) {
        prettyTimestamp = [NSString stringWithFormat:@\"%d days ago\", (int) floor(delta/86400.0) ];
    } else {
        NSDateFormatter * formatter = [[NSDateFormatter alloc] init];
        [formatter setDateStyle:NSDateFormatterMediumStyle];

        prettyTimestamp = [NSString stringWithFormat:@\"on %@\", [formatter stringFromDate:self]];
        [formatter release];
    }

    return prettyTimestamp;
}

- (NSString*)formatShortRelativeTime;