Calculating Coordinated Mars Time v2.0 - ios

Question

This is the follow up of a previous question of mine.
In a nutshell, I am trying to follow this tutorial step-by-step: https://jtauber.github.io/mars-clock/ to get to Coordinated Mars Time, but I got stuck right before the end. My code works fine up until the end (some values are more accurate than in the tutorial because I went back to the source from NASA: https://www.giss.nasa.gov/tools/mars24/help/algorithm.html ):
double millis = ( [[NSDate date] timeIntervalSince1970] * 1000 );
NSLog(#"millis: %f", millis);
double JDUT = ( 2440587.5 + (millis / 86400000) );
NSLog(#"JDUT: %f", JDUT);
double JDTT = ( JDUT + (37 +32.184) / 86400);
NSLog(#"JDTT: %f", JDTT);
double J2000Epoch = ( JDTT - 2451545.0 );
NSLog(#"J2000Epoch: %f", J2000Epoch);
double MSD = ( (( J2000Epoch - 4.5 ) / 1.0274912517) + 44796.0 - 0.0009626 );
NSLog(#"MSD: %f", MSD);
The only step remaining is actually calculating Coordinated Mars Time, using this equation:
MTC = mod24 { 24 h × MSD }
The problem is that I have no idea how. I tried to use modf( (double), (double *) ) but no idea how it actually works. I tried it the way below, but it gave me an incorrect answer (obviously as I have really no idea what I am doing). :(
double MSD24 = (24 * MSD);
double MCT = modf(24, &MSD24);
NSLog(#"MCT: %f", MCT); // Result: 0.000000
Any help would be much appreciated. Thank you very much!
p.s.: Notice that I use Objective-C; I do not understand swift unfortunately! :(

Answer 1

Carrying on from the code you gave, I tried:
CGFloat MTC = fmod(24 * MSD, 24);
and got
// 19.798515
which was right according to the web page you cited at the moment I tried it.
The sort of thing his page actually shows, e.g. "19:49:38" or whatever (at the time I tried it), is merely a string representation of that number, treating it as a number of hours and just dividing it up into minutes and seconds in the usual way. Which, I suppose, brings us to the second part of your question, i.e. how to convert a number of hours into an hours-minutes-seconds representation? But that is a simple matter, dealt with many times here. See NSNumber of seconds to Hours, minutes, seconds for example.
So, carrying on once again, I tried this:
CGFloat secs = MTC*3600;
NSDate* d = [NSDate dateWithTimeIntervalSince1970:secs];
NSDateFormatter* df = [NSDateFormatter new];
df.dateFormat = #"HH:mm:ss";
df.timeZone = [NSTimeZone timeZoneWithAbbreviation:#"GMT"];
NSString* result = [df stringFromDate:d];
NSLog(#"%#", result); // 20:10:20
...which is exactly the same as his web page was showing at that moment.
And here's a Swift version for those who would like to know what the "mean time" is on Mars right now:
let millis = Date().timeIntervalSince1970 * 1000
let JDUT = 2440587.5 + (millis / 86400000)
let JDTT = JDUT + (37 + 32.184) / 86400
let J2000Epoch = ( JDTT - 2451545 )
let MSD = (( J2000Epoch - 4.5 ) / 1.0274912517) + 44796.0 - 0.0009626
let MTC = (24 * MSD).truncatingRemainder(dividingBy: 24)
let d = Date(timeIntervalSince1970: MTC*3600)
let df = DateFormatter()
df.dateFormat = "HH:mm:ss"
df.timeZone = TimeZone(abbreviation: "GMT")!
df.string(from:d)