The basic format of DateInterval begins with P and contains the year, month, day, and time. It must be written in order and the corresponding letters must be used. The standard format is P year Y[D]T hour H[second S]. For example, P1D represents one day, and PT1H30M represents one hour and thirty minutes. Common errors include the lack of P, the order is reversed, the time part is missing T, and spaces or symbols before the number. When used in actual use, DateInterval usually performs date addition and subtraction operations with the DateTime object, and can also be used to traverse date ranges. In addition, it supports the rapid creation of common intervals such as daily, weekly, and monthly, but does not support logic such as "one day of the month" and requires additional processing. Mastering format rules is the key to using DateInterval correctly.

The DateInterval class in PHP is used to represent time intervals and is often used for date addition and subtraction operations. However, many people are not very clear about its formatting rules when using it, especially when constructing strings, which are prone to errors.

In fact, this format has a fixed way of writing, the key is to remember the letters and order corresponding to each unit.

What is the basic format?

The constructor of DateInterval accepts a string parameter, and the format must start with P , representing "Time period". If the time part is included, you also need to add T to separate the date and time.

The standard format is as follows:

 P[Year Y][Month M][Day D]T[Hour H][Minutes M][seconds S]

For example:

• P1D means one day
• P2Y4M3D represents two years, four months and three days
• PT1H30M means one hour and thirty minutes
• P1Y2M3DT4H5M6S represents two months a year, three days, four hours, five minutes and six seconds

Note: T in the middle is the start sign of the time part and cannot be missed.

Common Errors and Precautions

Although some formats look reasonable, PHP will report errors or parse incorrectly. Here are some FAQs:

• Forgot to add P : For example, write it 1D , which will lead to an error.
• The order is reversed : it must be written in the order of year, month, day and time, and cannot be changed.
• Ignore T : For example, P1H is illegal because H belongs to the time part and should be written as PT1H .
• There cannot be spaces or symbols before a number : it can only be pure numerical unit letters.

For example:

 $interval = new DateInterval('P1D'); // Correct $interval = new DateInterval('1D'); // Error, P is missing
$interval = new DateInterval('PT60M'); // Correct: 60 minutes

How to actually use DateInterval?

Usually we will use it with the DateTime object to add and subtract dates.

For example, add one day to the current date:

 $date = new DateTime();
$interval = new DateInterval('P1D');
$date->add($interval);
echo $date->format('Ym-d');

You can also use it to iterate over the date range:

 $start = new DateTime('2024-01-01');
$end = new DateTime('2024-01-10');

$interval = new DateInterval('P1D');
$period = new DatePeriod($start, $interval, $end);

foreach ($period as $dt) {
    echo $dt->format("Ymd") . "\n";
}

This makes it easy to list all dates from 1 to 9.

Tips: Quickly create common intervals

If you just want to simply increase the number of days, hours, etc., you can directly generate it with strings:

• Add one day: P1D
• Three hours reduction: PT-3H
• Once a week: P7D or P1W
• The first day of each month: You can set it to the first day of the month and then add P1M

However, it should be noted that DateInterval does not support logic like "one day of every month" and needs to be processed with additional code.

Basically that's it. The format looks strange, but I get used to it after using it a few times. The key is to write it according to the rules, otherwise PHP will not be able to parse it.

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