javascriptで月末日を求める

12+1=13になるけど、そこも問題ないみたい、0が1月らしい

var n = new Date('2019-12-01')
var m = new Date(n.getFullYear(),n.getMonth()+1,0);
console.log(m);

> Tue Dec 31 2019 00:00:00 GMT+0900 (日本標準時)


sql DATE_ADD

select DATE_ADD('2019-01-31', INTERVAL + 1 MONTH);
2019-02-28

select DATE_ADD('2019-01-30', INTERVAL + 1 MONTH);
2019-02-28

select DATE_ADD('2019-01-29', INTERVAL + 1 MONTH);
2019-02-28

select DATE_ADD('2019-01-28', INTERVAL + 1 MONTH);
2019-02-28

select DATE_ADD('2019-01-27', INTERVAL + 1 MONTH);
2019-02-27

同じ時刻じゃないの？！！という罠


?cdate("2014/01/02 16:05:00") = dateadd("n",5,cdate("2014/01/02 16:00:00"))
False
?cdate("2014/01/02 16:05:00") - dateadd("n",5,cdate("2014/01/02 16:00:00"))
7.27595761418343E-12

おかしいでしょう！ 16:05:00は16:00:00に5分足すじゃない！
参考ここ：
https://www.reddit.com/r/vba/comments/5mdnpb/pic_of_my_data_mismatch_problem/
http://www.fmsinc.com/tpapers/math/index.html

DateDiff("s", cdate("2014/01/02 16:05:00"), dateadd("n",5,cdate("2014/01/02 16:00:00"))) = 0


mysqlの日付の比較メソッド,datediffとperiod_diff


select abs(datediff('2013-01-01','2014-01-01'));
#365
select period_diff(date_format('2014-03-06', '%Y%m'), date_format('2012-03-06', '%Y%m')) as month;
#24


ruby date 比較


$ irb
irb(main):001:0> require 'date'
=> true
irb(main):002:0> from = Date::strptime('2013-01-01', '%Y-%m-%d')
=> #
irb(main):004:0> to = Date::strptime('2013-01-02', '%Y-%m-%d')
=> #
irb(main):005:0> same = Date::strptime('2013-01-02', '%Y-%m-%d')
=> #
irb(main):006:0> from > to
=> false
irb(main):007:0> from < to => true
irb(main):008:0> to == same
=> true