Question

In my app I would like to allow the user to enter a value only once a day.

How do I make sure the the stored NSDate is not within the current day? Can I compare the day value of two NSDate while ignoring the time?

Answer 1

Dates are just wrappers around NSTimeIntervals. They have no concept of local time, so you can’t get a day value out of them directly.

You could use NSDateFormatter, like this:

NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
[formatter setDateFormat:@"yyyy/MM/dd"];

NSString *today = [formatter stringFromDate:[NSDate date]];

NSString *dateToCompare = [formatter stringFromDate:someDate];

if ([dateToCompare isEqualToString:today])
{
    ...
}

Date formatters output the date using your local timezone, so you can guarantee that the day will be correct for the user.

If you actually want to ensure that the events are more than 24 hours apart (to prevent cheating) it’s much easier, just do this:

if ([date1 timeIntervalSinceDate:date2] > 3600*24)
{
    ...
}

UPDATE: As pointed out by abbood in the comments, it can be expensive/slow to create NSDateFormatter objects, so you should hang onto them if they will be reused.

It isn’t a good idea to store them in a static variable or singleton because they are not thread-safe, but you can store them in the current thread’s threadDictionary, like this:

//check for a cached formatter and recreate if needed
NSMutableDictionary *threadDict = [[NSThread currentThread] threadDictionary];
NSDateFormatter *formatter = threadDict[@"someUniqueKey"];
if (!formatter)
{
    //create the formatter
    formatter = [[NSDateFormatter alloc] init];
    [formatter setDateFormat:@"yyyy/MM/dd"];

    //cache the formatter
    threadDict[@"someUniqueKey"] = formatter;
}

//use the formatter
...