TimeInterval calculation issue - objective-c

Question

I am trying to create an app that calculates the time difference and the amount is multiplied by an amount in money. It is targeted to calculate in R$ (brazilian real) the amount that someone will have to pay for using a service during the time calculated by the app.
Here´s is my code:
- (IBAction)encerrar:(id)sender {
NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
dateFormatter.dateFormat = #"HH:mm";
NSString *temp = inicio.text;
NSDate *then = [dateFormatter dateFromString:temp];
NSDate *now = [NSDate date];
NSTimeInterval timeInterval = [now timeIntervalSinceDate:then];
NSString *ext = [dateFormatter stringFromDate:now];
fim.text = ext;
NSDate *timerDate = [NSDate dateWithTimeIntervalSince1970:timeInterval];
[dateFormatter setTimeZone:[NSTimeZone timeZoneForSecondsFromGMT:0.0]];
NSString *timeString = [dateFormatter stringFromDate:timerDate];
duracao.text = timeString;
double timeIntervalInHours = (timeInterval / 3600.0) * 5;
NSString *vTotal = [[NSString alloc] initWithFormat:#"R$ %.2f", fecha];
NSLog(#"%.2f", timeIntervalInHours);
vlrTotal.text = vTotal;
}
The fact is that when we click on Calculate button, and if the duracao (duration) is equal to 1h it gives me the correct amount which is R$ 5,00. But when the duration is equal to 30mins or other value different from exact 1h, it gives the wrong amount.
I.E.: 1h should be R$ 5,00; while 1:30h should be R$ 7,50, but shows me R$ 6,50.
So, anyone could help me on this???
Thanks in advance!!!

Answer 1

timeInterval is the amount of time in seconds, so if you want it in hours and fractions of an hour just do this:
double timeIntervalInHours = timeInterval / 3600.0;
and then multiple timeIntervalInHours times the price/hour to get the cost.
EDIT
Based on our chat, I would create a button with "iniciar" (start) as the title. When they press that button, I would store the current time and change the title to "encerrar" (stop). (I hope that my translations are correct, lol) Then the button action would look something like this:
// self.startTime is a NSDate.
- (IBAction)iniciar_encerrar:(UIButton *)sender {
if ([sender.titleLabel.text isEqualToString:#"iniciar"]) {
// We are starting the time
if (self.startTime != nil) {
return;
}
self.startTime = [NSDate date];
[sender setTitle:#"encerrar" forState:UIControlStateNormal];
} else {
// We are stopping the time
NSDate *currentTime = [NSDate date];
NSTimeInterval elapsedTimeInSeconds = [currentTime timeIntervalSinceDate:self.startTime];
double cost = elapsedTimeInSeconds / 3600.0 * 5.0;
NSLog(#"%.2lf", cost);
// reset the button
self.startTime = nil;
[sender setTitle:#"iniciar" forState:UIControlStateNormal];
}
}
startTime is declared as follows:
In your .h file, along with the other declared properties, add:
#property (strong, nonatomic) NSDate *startTime;
In your .m file, add this at the top with the other ones:
#synthesize startTime;
In your viewDidUnload function (in your .m file) add this:
startTime = nil;
This just gives you a place to store the startTime date.

Answer 2

You're converting the time string HHmm to a float, so at 1.5 hours, you will have a float of 130 (1 hour 30 min). I can't follow your calculations from there. I think what you want is to have two formatters, one to get minutes, and one for hours, like so:
formatter.dateFormat = #"HH";
float hours = [[formatter stringFromDate:timerDate] intValue] * 1.0;
formatter.dateFormat = #"mm";
hours += [[formatter stringFromDate:timerDate] intValue]/60.0; //convert min to fractional hours and add

Answer 3

I would convert the amount $R5,00 from an hourly amount to an amount per second for better accuracy. 5/ 60 minutes / 60 seconds gives you .00388889 which you would multiply by (duration*1000) to give you your accurate result.