Confusion about NSTimeZone.secondsFromGMT - ios

Question

I am developing an app that has a feature to enter dark/night mode during night hours automatically. The app asks for the user location and determines the sunrise/sunset hour (in Universal Time) using this algorithm.
The only step that is not clear is to convert from UT to local time, since this is not explained in the algorithm. Say I get a sunrise time of 8.5 (8:30 in the morning UT). How could I convert it to user's local time to check if it's day or night? Or equivalently, how could I convert user's local time to UT in order to be able to compare them?
So far I've tried to use NSCalendar to get the NSDateComponents of the current date (NSDate()). One of these components is a NSTimeZone? from which I can get the secondsFromGMT. Something like this:
let dateComponents = NSCalendar.currentCalendar().components([.TimeZone], fromDate: NSDate())
let localOffset = Double(dateComponents.timeZone?.secondsFromGMT ?? 0)/3600
where localOffset should be the time difference (in hours) from UT (i.e. GMT if I am right) to local time, defaulting to 0 if dateComponents.timeZone == nil (I don't know under which situations this could happen). The problem is that I get the same localOffset for now than for 6 months in the future (when the daylight saving time will be different than it is now at my location, Spain). Does this mean that I need to use the properties daylightSavingTime and/or daylightSavingTimeOffset together with secondsFromGMT? Doesn't secondsFromGMT itself account for this?
Things get even more confusing to me when I read the results from the algorithm. The sun setting hour (in local time) is exactly the one given by Google, but the sun rising hour is one hour ahead of what Google says (for my location and date). I share with you the whole Swift implementation of the algorithm hoping that it can help someone spot what's that I'm doing wrong.
import Foundation
import CoreLocation
enum SunriseSunsetZenith: Double {
case Official = 90.83
case Civil = 96
case Nautical = 102
case Astronomical = 108
}
func sunriseSunsetHoursForLocation(coordinate: CLLocationCoordinate2D, atDate date: NSDate = NSDate(), zenith: SunriseSunsetZenith = .Civil) -> (sunrise: Double, sunset: Double) {
// Initial values (will be changed later)
var sunriseTime = 7.5
var sunsetTime = 19.5
// Get the longitude and latitude
let latitude = coordinate.latitude
let longitude = coordinate.longitude
// Get the day, month, year and local offset
let dateComponents = NSCalendar.currentCalendar().components([.Day, .Month, .Year, .TimeZone], fromDate: date)
let day = Double(dateComponents.day)
let month = Double(dateComponents.month)
let year = Double(dateComponents.year)
let localOffset = Double(dateComponents.timeZone?.daylightSavingTimeOffset ?? 0)/3600
// Calculate the day of the year
let N1 = floor(275*month/9)
let N2 = floor((month + 9)/12)
let N3 = 1 + floor((year - 4*floor(year/4) + 2)/3)
let dayOfYear = N1 - N2*N3 + day - 30
for i in 0...1 {
// Convert the longitude to hour value and calculate an approximate time
let longitudeHour = longitude/15
let t = dayOfYear + ((i == 0 ? 6.0 : 18.0) - longitudeHour)/24
// Calculate the Sun's mean anomaly
let M = 0.9856*t - 3.289
// Calculate the Sun's true longitude
var L = M + 1.916*sind(M) + 0.020*sind(2*M) + 282.634
L %= 360
// Calculate the Sun's right ascension
var RA = atand(0.91764 * tand(L))
RA %= 360
let Lquadrant = (floor(L/90))*90
let RAquadrant = (floor(RA/90))*90
RA += Lquadrant - RAquadrant
RA /= 15
// Calculate the Sun's declination
let sinDec = 0.39782*sind(L)
let cosDec = cosd(asind(sinDec))
// Calculate the Sun's local hour angle
let cosH = (cosd(zenith.rawValue) - sinDec*sind(latitude))/(cosDec*cosd(latitude))
if cosH > 1 { // The sun never rises on this location (on the specified date)
sunriseTime = Double.infinity
sunsetTime = -Double.infinity
} else if cosH < -1 { // The sun never sets on this location (on the specified date)
sunriseTime = -Double.infinity
sunsetTime = Double.infinity
} else {
// Finish calculating H and convert into hours
var H = ( i == 0 ? 360.0 : 0.0 ) + ( i == 0 ? -1.0 : 1.0 )*acosd(cosH)
H /= 15
// Calculate local mean time of rising/setting
let T = H + RA - 0.06571*t - 6.622
// Adjust back to UTC
let UT = T - longitudeHour
// Convert UT value to local time zone of latitude/longitude
let localT = UT + localOffset
if i == 0 { // Add 24 and modulo 24 to be sure that the results is between 0..<24
sunriseTime = (localT + 24)%24
} else {
sunsetTime = (localT + 24)%24
}
}
}
return (sunriseTime, sunsetTime)
}
func sind(valueInDegrees: Double) -> Double {
return sin(valueInDegrees*M_PI/180)
}
func cosd(valueInDegrees: Double) -> Double {
return cos(valueInDegrees*M_PI/180)
}
func tand(valueInDegrees: Double) -> Double {
return tan(valueInDegrees*M_PI/180)
}
func asind(valueInRadians: Double) -> Double {
return asin(valueInRadians)*180/M_PI
}
func acosd(valueInRadians: Double) -> Double {
return acos(valueInRadians)*180/M_PI
}
func atand(valueInRadians: Double) -> Double {
return atan(valueInRadians)*180/M_PI
}
Ans this is how I use the function to determine if it's night or not:
let latitude = ...
let longitude = ...
let coordinate = CLLocationCoordinate2D(latitude: latitude, longitude: longitude)
let (sunriseHour, sunsetHour) = sunriseSunsetHoursForLocation(coordinate)
let componetns = NSCalendar.currentCalendar().components([.Hour, .Minute], fromDate: NSDate())
let currentHour = Double(componetns.hour) + Double(componetns.minute)/60
let isNight = currentHour < sunriseHour || currentHour > sunsetHour

Answer 1

I'm not sure why your code to get the offset isn't working (I got the same result). But there's a simpler solution that does work. Just ask the local time zone, using secondsFromGMTForDate. With dates six months apart I get different results:
let now = NSDate()
let future = NSCalendar.currentCalendar().dateByAddingUnit(NSCalendarUnit.Month, value: 6, toDate: now, options: NSCalendarOptions(rawValue: 0))!
let nowOffset = NSTimeZone.localTimeZone().secondsFromGMTForDate(now)/3600
let futureOffset = NSTimeZone.localTimeZone().secondsFromGMTForDate(future)/3600