php Carbon - 为什么 addMonths() 改变月份中的某一天？
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Carbon - why addMonths() change the day of month?
提问by Limon Monte
Here's the simple example (today is 2016-08-29):
这是一个简单的例子（今天是 2016-08-29）：
var_dump(Carbon::now());
var_dump(Carbon::now()->addMonths(6));
Output:
输出：
object(Carbon\Carbon)#303 (3) {
["date"] => string(26) "2016-08-29 15:37:11.000000"
}
object(Carbon\Carbon)#303 (3) {
["date"] => string(26) "2017-03-01 15:37:11.000000"
}
For Carbon::now()->addMonths(6)I'm expecting 2017-02-29, not 2017-03-01.
因为Carbon::now()->addMonths(6)我在期待2017-02-29，而不是2017-03-01。
Am I missing something about date modifications?
我是否缺少有关日期修改的信息？
回答by RIanGillis
There is no 2/29/2017. Leap-year happened in 2016.
没有 2/29/2017。闰年发生在 2016 年。
The following link:
以下链接：
http://carbon.nesbot.com/docs/#api-addsub
http://carbon.nesbot.com/docs/#api-addsub
provides an example of adding 1 month to 1/31/2012 and arriving on 3/3/2012. Which is intended, though seems confusing to me.
As a counter-example exhibiting different behavior, in SQL:
提供了将 1 个月添加到 2012 年 1 月 31 日并于 2012 年 3 月 3 日到达的示例。这是有意的，尽管对我来说似乎很困惑。
作为表现出不同行为的反例，在 SQL 中：
SELECT dateadd(m,1,'2012-01-31')
results in 2/29/2012, so it would be a good idea to check the specifications of whatever addmonth() function you plan on using.
结果是 2/29/2012，因此检查您计划使用的任何 addmonth() 函数的规格是个好主意。
回答by Alexander Malakhov
It's even more crippled than that - subtraction has same issues. There are special methods to avoid overflows, though:
它甚至比那更残缺 - 减法也有同样的问题。不过，有一些特殊的方法可以避免溢出：
function original(){
return new Carbon('2016-08-31');
};
function print_dt($name, $date){
echo $name . $date->toAtomString() . PHP_EOL;
};
print_dt('original: ', original());
echo '-----' . PHP_EOL;
print_dt('addMonths: ', original()->addMonths(6));
print_dt('addMonthsNoOverflow: ', original()->addMonthsNoOverflow(6));
echo '-----' . PHP_EOL;
print_dt('subMonths: ', original()->subMonths(2));
print_dt('subMonthsNoOverflow: ', original()->subMonthsNoOverflow(2));
output:
输出：
original: 2016-08-31T00:00:00+00:00
-----
addMonths: 2017-03-03T00:00:00+00:00
addMonthsNoOverflow: 2017-02-28T00:00:00+00:00
-----
subMonths: 2016-07-01T00:00:00+00:00
subMonthsNoOverflow: 2016-06-30T00:00:00+00:00
