DateTime::add()

date_add

(PHP 5 >= 5.3.0, PHP 7)

给一个 DateTime 对象增加一定量的天，月，年，小时，分钟以及秒。

说明

将给定的DateInterval对象加到DateTime对象上。

参数

仅过程化风格：由date_create()返回的DateTime类型的对象。此函数会修改这个对象。

DateInterval对象。

返回值

返回被修改的 DateTime 对象，或者在失败时返回FALSE.

范例

Example #1DateTime::add()例程

面向对象风格

过程化风格

以上例程会输出：

2000-01-11

FurtherDateTime::add()例程

以上例程会输出：

2000-01-01 10:00:30
2007-06-05 04:03:02

当在 DateTime 上加月的时候要注意

以上例程会输出：

2001-01-31
2001-03-03

注释

在 PHP 5.2 的版本中，也可以使用DateTime::modify()方法来替代本方法。

参见

• DateTime::sub() 对一个 DateTime 对象减去一定量的日、月、年、小时、分钟和秒。
• DateTime::diff() Returns the difference between two DateTime objects
• DateTime::modify() 修改日期时间对象的值

Note that the add() and sub() methods will modify the value of the object you're calling the method on! This is very untypical for a method that returns a value of its own type. You could misunderstand it that the method would return a new instance with the modified value, but in fact it modifies itself! This is undocumented here. (Only a side note on procedural style mentions it, but it obviously does not apply to object oriented style.)

Another simple solution to adding a month but not autocorrecting days to the next month is this.
(Also works for substracting months)
$dt = new DateTime("2016-01-31");
$oldDay = $dt->format("d");
$dt->add(new DateInterval("P1M")); // 2016-03-02
$newDay = $dt->format("d");
if($oldDay != $newDay) {
  // Check if the day is changed, if so we skipped to the next month.
  // Substract days to go back to the last day of previous month.
  $dt->sub(new DateInterval("P" . $newDay . "D"));
}
echo $dt->format("Y-m-d"); // 2016-02-29
Hope this helps someone.

If you're using PHP >= 5.5, instead of using "glavic at gmail dot com"'s DateTimeEnhanced class, use the built in DateTimeImmutable type. When you call DateTimeImmutable::add() it will return a new object, rather than modifying the original

Here is a solution to adding months when you want 2014-10-31 to become 2014-11-30 instead of 2014-12-01. 

If you need add() and sub() that don't modify object values, you can create new methods like this: 

If you use fraction of seconds, you may have surprises. It only occurs when the sum of the floating point parts results in exactly 1 second (0.5 + 0.5 ou 0.3 + 0.7, for example). See these cases at intervals slightly bigger than 1 second:

$strDataHora is correct: "2017-12-31T23:59:59.900"

$strDataHora is correct: "2018-01-01T00:00:00.100"
But...

$strDataHora has "2017-12-31T23:59:59.1000"
To resolve, add 1 second to the interval and f property must be negative (-1.0 plus original value):

$strDataHora is correct: "2018-01-01T00:00:00.000"

What you can do with this function/method is a great example of the philosophy: "just because you can do it doesn't mean you should". I'm talking about two issues: (1) the number of days in the month which varies from months 1-12 as well as for month 2 which could be leap year (or not); and then issue (2): what if there is the need to specify a large quantity of an interval such that it needs to be re-characterized into broader-scoped intervals (i.e. 184 seconds ==> 3 minutes-4 seconds). Examples in notes elsewhere in the docs for this function illustrate both issues and their undesired effects so I won't focus on them further. But how did I decide to handle? I've gone with four "public" functions and a single "private" function, and without giving you a bunch of code to study, here are their summaries...
1. function adjustYear(int $yearsAdj){ //you can pass in +/- value and I adjust year value by that value but then I also call PHP's 'cal_days_in_month' function to ensure the day number I have in my date does not exceed days in the month for the new year/month combo--if it does, I adjust the day value downward.
2. function adjustMonth(int $monthsAdj){ //same notes as above apply; but also, I allow any number to be passed in for $monthsAdj. I use the 'int' function (int($monthsAdj/12)) and modulus % operator to determine how to adjust both year and month. And again, I use 'cal_days_in_month' function to tweak the day number as needed.
3. function addTime(int $days, int $hours, int $minutes, int $seconds){
// I use date_add and create a DateInterval object from the corresponding string spec (created from the args passed to this function). Note that months and years are excluded due to the bad side-effects already mentioned elsewhere.
4. function subtractTime(int $days, int $hours, int $minutes, int $seconds){
//notes for "addTime" also apply to this function but note that I like separate add and subtract functions because setting the DateInterval property flag to indicate add/subtract is not as intuitive for future coding.
5. function recharacterizeIntervals(int $days, int $hours, int $minutes, int $seconds){ // I convert excessively large quantities of any one interval into the next largest interval using the 'int' function and modulus (%) operator. I then use the result of this function when creating the string interval specification that gets passed when generating the DateInterval object for calling the date_add function (or object-method equivalent).
**Results/goals...
--any number of days/hours/minutes/seconds can be passed in to add/subtractTime and all of "Y/M/D/H/M/S" values get adjusted as you would expect.
--using adjustYear/Month lets you pass +/- values and only "Y/M" values get modified without having undesirable effects on day values.
--a call to the "recharacterize" function helps ensure proper and desired values are in the intervals prior to calling date_add to let it do its work.

Be careful when using this function, I may have happened upon a bug in PHP7.
My code is as follows
//get date from post or else fill with today's date
if (isset($_POST["from"]))
{
$from = date_create($_POST["from"]);
}else{
$from = date_create(date("Y-m-d"));
}
//get date from post if there isn't one just take the same date as what is in the $from variable and add one day to it
if (isset($_POST["to"]))
{
$to = date_create($_POST["to"]);
}else{
  $to = $from;
date_modify($to, '+1 day');
}
echo(date_format($from, 'Y-m-d') . " " . date_format($to, 'Y-m-d'));
The resultant output is
$from = 2015-12-11
$to = 2015-12-11
In actuality the result should be
$from = 2015-12-10
$to = 2015-12-11
For some reason the code above modifies the $from variable in the line date_modify($to, '+1 day'); even though it shouldn't as the $from variable isn't being modified.
to fix this i needed to change the code to
//get date from post or else fill with today's date
if (isset($_POST["from"]))
{
$from = date_create($_POST["from"]);
}else{
$from = date_create(date("Y-m-d"));
}
//get date from post if there isn't one just take the same date as what is in the $from variable and add one day to it
if (isset($_POST["to"]))
{
$to = date_create($_POST["to"]);
}else{
  $to = date_create(date("Y-m-d"));
date_modify($to, '+1 day');
}
echo(date_format($from, 'Y-m-d') . " " . date_format($to, 'Y-m-d'));
This isn't strictly the code I wanted. Possible bug?

adding 15 min to a datetime

period:
P1Y2M3DT1H2M3S
period time:
PT1H2M3S

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