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?
