Question

I’m trying to find the time interval since last move in a game. Normally, in objective-c, I do this and it works great:

    NSDate *now = [NSDate date]; 
    NSDate *before = [NSDate dateWithTimeInterval:-60 sinceDate:now]; 

    NSTimeInterval interval = [now timeIntervalSinceDate: before]; 

I store the lastTurn date in mysql database:

timestamp   -   ON UPDATE CURRENT_TIMESTAMP

When I try to get the date in objective-c, it seems to work:

    NSDate *lastTurn = match.lastTurn; 
    NSLog(@"lastTurn: %@", lastTurn);

    Output: lastTurn: 2012-03-21 09:40:32

…however, when I try to do a time interval, I get the -[__NSCFString timeIntervalSinceReferenceDate]: unrecognized selector sent to instance error.

    NSDate *lastTurn = match.lastTurn; 
    NSTimeInterval interval = [[NSDate date] timeIntervalSinceDate: lastTurn]; 

Any suggestions would awesome. Thanks

EDIT

I use ASIHTTPRequest to get the information from the database to the iphone.

The Match class is just a simple class that holds the variables:

@property (nonatomic, strong) NSDate *lastTurn; 
etc...

… And I get the variable by doing this:

    [request setCompletionBlock:^
    {

     for (NSDictionary *dict in responseDict)
     {                
             Match *match = [[Match alloc] init];  
             match.lastTurn = [dict objectForKey:@"lastTurn"]; 

Answer 1

(disclaimer: I know nothing about objective-c, so this might be complete rubbish, only reading the documentation leads to dangerous superficial knowledge ;-))

My bets are on

NSDate *lastTurn = match.lastTurn;
NSLog(@"className of lastTurn is %@", [lastTurn className]);

prints className of lastTurn is __NSCFString and you have to convert that string into an instance of NSDate via e.g. dateWithString before using it like an instance of NSDate.

---

NSDate Class Reference says:

The sole primitive method of NSDate, timeIntervalSinceReferenceDate, provides the basis for all the other methods in the NSDate interface.

Which means that timeIntervalSinceReferenceDate is called on both [NSDate date] and lastTurn.
The error message -[__NSCFString timeIntervalSinceReferenceDate] states (if I’m not mistaken) that the selector timeIntervalSinceReferenceDate has been sent to an instance of __NSCFString, i.e. a “flavour” of NSString, not an instance of NSDate.
Since [NSDate date] is most likely an instance of NSDate there’s only lastTurn left.

My guess is that all entries in responseDict are strings and a simple NSDate *lastTurn = match.lastTurn; doesn’t do any conversion, not even (duck-)type-checking.