NSString+NSTimeInterval.m

/// Take an NSTimeInterval and return a string representation in the format h:mm:ss
+ (NSString *)formatWithInterval: (NSTimeInterval)interval
{
    NSAssert(interval >= 0, @"interval must be >= 0");
    unsigned long seconds = interval;
    unsigned long minutes = seconds / 60;
    seconds %= 60;
    unsigned long hours = minutes / 60;
    minutes %= 60;
    NSMutableString *result = [NSMutableString string];
    
    if(hours)
        [result appendFormat: @"%d:", hours];
    
    [result appendFormat: @"%02d:", minutes];
    [result appendFormat: @"%02d", seconds];
    return result;
}