NSDateから文字列生成
-(NSString*) getDatetimeFromDate:(NSDate*)date
{
    NSDateFormatter* form = [[NSDateFormatter alloc] init];
    NSCalendar* cal = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
    [form setCalendar: cal];
    [form setDateFormat:@"yyyy-MM-dd HH:mm:ss"];
    NSString* str = [form stringFromDate:date];
    return str;
}
NSString*dateString=[self getDatetimeFromDate:[NSDate date]];