日期转换

NSDate *  <--> NSString *

主要使用  NSDateformater  进行格式转换

NSDateFormatter * formatter = [[NSDateFormatter alloc]init];

formatter.dateFormat = @"yyyy-MM-dd HH:mm:ss";

//  设置语言区域(因为这种时间是欧美常用时间)
formatter.locale = [[NSLocale alloc] initWithLocaleIdentifier:@"en_US"];
   
NSDate * date = [NSDate date];
NSString * dateString = [formatter stringFromDate:date];
    
NSString * string = @"2016-07-19 10:25:07";
NSDate * stringDate = [formatter dateFromString:string];

NSCalendar * calendar;

if ([UIDevice currentDevice].systemVersion.doubleValue >= 8.0) {
   calendar = [NSCalendar calendarWithIndentifier:NSCalendarIdentifierGregorian];
} else {
  calendar = [NSCalendar currentCalendar];
}

NSString * string = @"123456787654754";
NSTimeInterval second = string.longLongValue / 1000.0;
NSDate * date = [NSDate dateWithTimeIntervalSince1970];

NSString *string = @"2015-11-20 09:10:05";   
NSString *month = [string substringWithRange:NSMakeRange(5, 2)];