NSTimeInterval timeInMiliseconds = [[NSDate date] timeIntervalSince1970];

要改变它,

NSDate* date = [NSDate dateWithTimeIntervalSince1970:timeInMiliseconds];

按照apple documentation,

NSTimeInterval：用于指定时间间隔(以秒为单位).

typedef double NSTimeInterval;

它是double型.

要将日期转换为字符串,

NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
[formatter setDateFormat:@"yyyy-MM-dd HH:mm:ss zzz"];

//Optionally for time zone converstions
[formatter setTimeZone:[NSTimeZone timeZoneWithName:@"..."]];

NSString *stringFromDate = [formatter stringFromDate:myNSDateInstance];

[formatter release];