dateWithString:format: returning nil

[evanrobertson]

If I try and create a date from a format string every so often it will return nil, whilst using dateWithYear:month:day: with the same parameters will return the correct value.

Unsure if this is a bug with the library, looking at the method it seems like it might be something I am doing wrong, however I would like to know what I may be doing wrong.

NSNumber *year = @(2015);
NSNumber *month = @(10);

NSString *dateString = [NSString stringWithFormat:@"01/%@/%@", month, year];
NSDate *key = [NSDate dateWithString:dateString formatString:@"dd/MM/yyyy"]; // Sometimes nil
NSDate *key2 = [NSDate dateWithYear:year month:month day:1]; // Always successful

[evanrobertson]

To answer my own question,
NSDateFormatter is thread-unsafe

As is NSCalendar, so I guess my next question is about thread safety in DateTools, can we make improvements to allow us to work with dates on multiple threads?

So it turns out that NSDateFormatter is thread safe as of iOS 7, however it seems that you shouldn't be changing values on it from different thread, i.e. format string and time zone

What should we do to handle the scenario here where we are explicitly changing those values and it could be across threads?