Math issue using timeIntervalSince 1970

Question

I am trying to find how many milliseconds into the current day we are. I can't find a method to return the time in milliseconds ignoring date, so I figured I could calculate it off of the value returned by timeIntervalSince 1970 method.

I did this:

NSLog(@"%f", [[NSDate date] timeIntervalSince1970]);
2013-05-21 16:29:09.453 TestApp[13951:c07] 1369171749.453490

Now my assumption is that, since there are 86,400 seconds in a day I could divide this value by 86400 and get how many days have elapsed since 1970. Doing this gives me 15846.8952483 days. Now, if my assumption holds, I am 89.52483% through the current day. So multiple 24 hours by 86.52659% would give me a current time of the 21.4859592 hour or about 09:29 PM. As you can see from my NSLog this is about 5 hours from the real time, but I believe the interval returned is GMT so this would be 5 hours ahead of my time zone.

So I figured, well what the heck, I'll just roll with it and see what happens.

I cut off the decimal places by doing:

float timeSince1970 = [[NSDate date] timeIntervalSince1970]/86400.0;
timeSince1970 = timeSince1970 - (int)timeSince1970

Then calculate the milliseconds that have taken place thus far today:

int timeNow = timeSince1970 * 86400000;
NSLog(@"%i", timeNow);
2013-05-21 16:33:37.793 TestApp[14009:c07] 77625000

Then I convert the milliseconds (which still seem appropriate) to NSDate:

NSString *timeString = [NSString stringWithFormat:@"%d", timeNow];
NSDateFormatter *dateFormatter = [[NSDateFormatter alloc]init];
[dateFormatter setDateFormat:@"A"]
NSDate *dateNow = [dateFormatter dateFromString:timeString];
NSLog(@"%@", dateNow);
2013-05-21 16:29:09.455 TestApp[13951:c07] 2000-01-02 03:29:00 +0000

And there is my problem. Rather than returning a 2000-01-01 date with some hours and minutes attached, it is returning a 2000-01-02 date. Why!?

EDIT

I got it working by "removing" the extra 5 hours I noted in the above with:

int timeNow = (timeSince1970 * 86400000) - (5 * 60 * 60 * 1000);

I don't understand why this is necessary though. If someone can explain I'd greatly appreciate it.

EDIT 2

Perhaps I should be asking a more elementary question about how to accomplish the task I'm trying to accomplish. I care about times (for example, 4pm is important but I could care less about the date). I've been storing these in NSDates created by:

[dateFormatter setDateFormat:@"hh:mm a"];
[dateFormatter dateFromString@"04:00 PM"];

All this seems to be going fine. Now I want to compare current time to my saved time and find out if it is NSOrderedAscending or NSOrderedDescending and respond accordingly. Is there a better way to be accomplishing this?

Answer 1

The part of your question that says that you want to calculate "how many milliseconds into the current day we are" and then "4pm is important but I could care less about the date" makes it not answerable.

This is because "today" there could have been a time change, which changes the number of milliseconds since midnight (by adding or subtracting an hour, for instance, or a leap second at the end of a year, etc....) and if you don't have the date, you can't determine the number of milliseconds accurately.

Now, to address your edited question: If we assume today's date, then you need to use the time that you have stored and combine it with today's date to get a "specific point in time" which you can compare to the current date and time:

NSString *storedTime = @"04:00 PM";

// Use your current calendar 
NSCalendar *cal = [NSCalendar currentCalendar];

// Create a date from the stored time
NSDateFormatter *dateFormatter = [NSDateFormatter new];
[dateFormatter setDateFormat:@"hh:mm a"];
NSDate *storedDate = [dateFormatter dateFromString:storedTime];

// Break it up into its components (ie hours and minutes)
NSDateComponents *storedDateComps = [cal components:NSHourCalendarUnit | NSMinuteCalendarUnit 
                                           fromDate:storedDate];

// Now we get the current date/time:
NSDate *currentDateAndTime = [NSDate date];

// Break it up into its components (the date portions)
NSDateComponents *todayComps = [cal components:NSYearCalendarUnit | NSMonthCalendarUnit | NSDayCalendarUnit
                                      fromDate:currentDateAndTime];

// Combine with your stored time
todayComps.hour = storedDateComps.hour;
todayComps.minute = storedDateComps.minute;

// Create a date from the comps.
// This will give us today's date, with the time that was stored
NSDate *currentDateWithStoredTime = [cal dateFromComponents:todayComps];

// Now, we have the current date and the stored value as a date, so it is simply a matter of comparing them:
NSComparisonResult result = [currentDateAndTime compare:currentDateWithStoredTime];

Answer 2

You need to use NSCalendar to generate NSDateComponents based on right now, then set the starting hour, minute, and second all to 0. That will give you the beginning of today. Then you can use NSDate's -timeIntervalSinceNow method to get back the time elapsed between now and your start date.

NSCalendar *cal = [NSCalendar currentCalendar];

NSDate *now = [NSDate date];

// BUILD UP NSDate OBJECT FOR THE BEGINNING OF TODAY
NSDateComponents *comps = [cal components: (NSYearCalendarUnit | NSMonthCalendarUnit | NSDayCalendarUnit) fromDate: now];
comps.hour = 0;
comps.minute = 0;
comps.second = 0;

// USE CALENDAR TO GENERATE NEW DATE FROM COMPONENTS
NSDate *startOfToday = [cal dateFromComponents: comps];

// YOUR ELAPSED TIME
NSLog(@"%f", [startOfToday timeIntervalSinceNow]);

Edit 1

If you're just looking to compare some NSDateObjects you can see if the time interval between then and now is negative. If so, that date is in the past.

NSDate *saveDate = [modelObject lastSaveDate];
NSTimeInterval difference = [saveDate timeIntervalSinceNow];
BOOL firstDateIsInPast = difference < 0;
if (firstDateIsInPast) {
    NSLog(@"Save date is in the past");
}

You could also use compare:.

NSDate* then = [NSDate distantPast];
NSDate* now = [NSDate date];
[then compare: now]; // NSOrderedAscending

Answer 3

it is returning a 2000-01-02 date. Why!?

Because your dateFormatter uses the current system locale's timezone.

If you insert ...

dateFormatter.timeZone = [NSTimeZone timeZoneForSecondsFromGMT:0];

... your date formatter will interpret the string correctly. But why not creating the date directly:

NSDate *dateNow = [NSDate dateWithTimeIntervalSinceReferenceDate:timeNow];