计算两个具有不同语言环境的NSD之间的时差

Question

I have a current NSString in the format of 2010-04-23 00:00:00 and then I'm trying to get the number of days passed from the current day. However, I'm not sure how to handle when the user changes their locale to Thailand for example.

Here is some of the code.

NSString *start = @"2010-04-23 00:00:00";
NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
NSDate *date = [formatter dateFromString:start];

//Region Format Thailand
NSDate *today = [[NSDate alloc] init];
NSTimeInterval difference =  [today timeIntervalSinceDate:start];
int numberOfDays = difference / 86400;

What would be the correct way to handle this situation so the number of days difference is accurate?

Answer 1

Dates are complicated.

If you want a difference in days, hours, minutes, seconds, that's easy: Convert everything to NSDate, calculate the difference in seconds, convert to days, hours, minutes, seconds.

Anything else, you need to first define what results you actually want. Today at 1am and 11pm is the same day, but today 11pm and tomorrow 1am are different days - even though in the first case the difference is 22 hours, in the second case just two hours. So you have to define what you want. You have to define for which case you want a result of "0 days" and for which case you want a result of "1 days".

And if you change time zones, some dates will move to a different day, some won't.

It's up to you to decide what result you want. In any case, I'd convert all dates to the relevant time zone, extract the day, and calculate days differences from that.

Answer 2

You need to convert the date into epoch time.
- (NSTimeInterval)timeIntervalSince1970

After you do that you can use the below code, to find the time difference in seconds and compare them.


NSDate* date1 = [NSDate date];
NSDate* date2 = [NSDate date];
NSTimeInterval secs = [date1 timeIntervalSinceDate:date2];

if (secs < 0)
{
NSLog("less");
}
else if (secs > 0)
{
NSLog("greater");
}
else
{
NSLog(@"same");
return NSOrderedSame;
}