如何将带有小时偏移量的时间戳（以秒为单位）转换为本地 iOS NSDate？

Question

Im trying to convert a timestamp in seconds since 01/01/1970 with an hour timezone offset (ie -6) to a local NSDate. Here is what i have so far:

    NSInteger startTime = 1312228800;
    NStInteger startTimeOffset = -6;
    NSDate *startTimeDate = [NSDate dateWithTimeIntervalSince1970:startTime];

    NSDateFormatter* dateFormatter = [[NSDateFormatter alloc] init];
    [dateFormatter setDateFormat:@"yyyy-MM-dd HH:mm:ss ZZZZ"];
    [dateFormatter setLocale:[NSLocale currentLocale]];        

    NSInteger gmtOffset = [startTimeOffset intValue]*60*60;
    [dateFormatter setTimeZone:[NSTimeZone timeZoneForSecondsFromGMT:gmtOffset]];        
    NSDate *newStartTimeDate = [dateFormatter dateFromString:[dateFormatter stringFromDate:startTimeDate]];

For example if i have the time of 1312228800 i get 08/01/2011 20:00 at GMT 0. The actual startTime is 08/01/2011 14:00 at GMT -6. However locally on the device if i am based in EST the startTime should be set to 08/01/2011 16:00 GMT -4.

However i just get back the same startTime with no offset taken into account which is 08/01/2011 20:00 in the example above.

Any ideas where im going wrong here, have a feeling its got something to do with the setDateFormat: argument?

thx

Answer 1

Do you need to use a formatter? What about something like this:

NSInteger offsetSecs = [startTimeOffset intValue] * 60 * 60;
NSDate *theDate = [NSDate dateWithTimeIntervalSince1970:(startTime + offsetSecs)];