NSTimeInterval, NSDate & int64_t Yields Unreasonable Time Calculation

Question

I am doing what should be a simple calculation involving things like int64_t, NSTimeInterval and NSDate in order to keep track of the expiration of a token which expires within an hour of its acquisition and is refreshed (exchanged for a new one) every 50 minutes.

The code I have in place works most of the time, but every once in a while, (for example) it calculates that the time that has passed since the last token exchange was 16,000 days. Obviously something is very wrong somewhere, but I am just not seeing it.

Here is the code:

1) Get a new token, its expiration is given in seconds until the token expires, which is always 3600 seconds, or 1 hour. I take this value in seconds and add it to the current time in terms of seconds since 1970 and store this as an int64_t value in core data:

NSTimeInterval secondsSince1970  = [[NSDate date] timeIntervalSince1970];
NSTimeInterval secondsUntilExpirationFrom1970 = secondsSince1970  + [[responseObject objectForKey:@"expires_in"]doubleValue];

account.tokenExp = [self.dataManager updateOAuthTokenExpirationDateForAccount:account tokenExpiration:secondsUntilExpirationFrom1970]; //method that saves to managed object context

2) Now, every time a need to use the token, I first check to see if it is within 10 minutes or 600 seconds of expiring. If it is, I request a new one and if not, I don't.

- (BOOL)tokenRequiresRefreshForAccount:(Account*)account
{
  NSTimeInterval secondsSince1970 = [[NSDate date] timeIntervalSince1970];
  NSTimeInterval secondsUntilTokenExpiration = account.tokenExp - secondsSince1970;
                  //account.tokenExp is the saved expiration of the token (a double) in core data

  NSLog(@"There are %f minutes until the token expires and we %@ the token.", secondsUntilTokenExpiration / 60, secondsUntilTokenExpiration < 600 ? @"will refresh" : @"will not refresh");

  return secondsUntilTokenExpiration < 600;
}

As I said, the above log most recently returned a value that equated to 16,000 days having passed since the last check, this was logged 3 minutes after a log that said that there were 45 minutes remaining until the token expiration.

If anyone can see my error or has a better approach for acquiring the same result, I'd very much appreciate it.

UPDATE

I've seen some posts about 'underflowing' a number variable. The variable "secondsUntilTokenExpiration" is occasionally a rather negative number...is it possible that I am underflowing this variable? I have tried storing it as an int64_t and an NSTimeInterval and both yield occasionally (very) incorrect values. If this could be the problem, how should I be storing this value?

Answer 1

The first thing that comes to mind is to make sure one of the function calls used to derive the value doesn't drop the accuracy (NSDate, round, responseObject...). It's a bit of a stretch, but that's all I can see at the moment.