How to offset NSDate with UTC timezone offset without hardcoded manual calculation

问题

Imagine the current local time being 15:11 UTC. I retrieve a data set from the server showing the opening closing time of a business displayed like so:

{
close = {
   day = 3;
   time = 0200;
};
open = {
   day = 2;
   time = 1700;
};

I also receive a utc-offset property exposed like so: "utc_offset" = "-420”; which I imagine is a minute offset giving an hour offset of 7 hours which seems right considering the timezone I'm in is UTC and the business location's opening hours information I'm receiving is for a business in Los Angeles who are 7 hours behind.

How do I use this property to then be able to do any time calculations on it I want to determine whether the current local time falls between the open and close time that bit I have figured out but the calculations come out wrong considering the time comparison is done in the local timezone when it needs to be offset before calculating against that time range.

I'm trying to avoid doing things like

Psuedocode: NSDate.date hour componenent + (UTC_offset / 60 = -7 hours)

Update: Here's how I'm currently checking if the business is open right now

        if currentArmyTime.compare(String(openInfo.time)) != .OrderedAscending && currentArmyTime.compare(String(closeInfo.time)) != .OrderedDescending {
            //The business is open right now, though this will not take into consideration the business's time zone offset.
        }

Is it easier to offset the current time?

回答1:

Before you can use the 'open' and 'close' times in date operations you need to create an NSDate from a calendar that has been set to the time zone for those times. Here's an example:

// Create calendar for the time zone
NSInteger timeOffsetInSeconds = -420 * 60;
NSTimeZone *tz = [NSTimeZone timeZoneForSecondsFromGMT:timeOffsetInSeconds];
NSCalendar *calendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSCalendarIdentifierGregorian];
calendar.timeZone = tz;

// Create an NSDate from your source data
NSDateComponents *comps = [[NSDateComponents alloc] init];
comps.day = 1;
comps.month = 1;
comps.year = 2016;
comps.hour = 8;
comps.minute = 0;
NSDate *openTime = [calendar dateFromComponents:comps];

// 'openTime' can now be to compared with local time.
NSLog(@"openTime = %@", openTime);  // Result is openTime = 2016-01-01 15:00:00 +0000

You should put the above code into a method that takes in the raw time and the time offset to apply.

来源：https://stackoverflow.com/questions/38457167/how-to-offset-nsdate-with-utc-timezone-offset-without-hardcoded-manual-calculati

标签

ios

swift

nsdate