json export/import date parsing is now done through extern blocks dateFromString  and stringFromDate that need to be implemented by app using mattgenerator, or you will get link errors:
naive unoptimized sample implementations could be these:
NSDate* (^dateFromString)(NSString* dateString, NSString* format) = ^NSDate* (NSString* dateString, NSString* format) {
    NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
    [formatter setTimeZone:[NSTimeZone timeZoneForSecondsFromGMT:0]];
    [formatter setLocale:[NSLocale currentLocale]];
    [formatter setDateFormat:format];
    return [formatter dateFromString:dateString];
};

NSString* (^stringFromDate)(NSDate* date, NSString* format) = ^NSString* (NSDate* date, NSString* format) {
    NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
    [formatter setTimeZone:[NSTimeZone timeZoneForSecondsFromGMT:0]];
    [formatter setLocale:[NSLocale currentLocale]];
    [formatter setDateFormat:format];

    NSString *dateString = [formatter stringFromDate:date];
    return dateString;
};